What exactly is your question? How to market the portals, what uses they could have or whether you overlooked some possibly catastrophic error in the system? As it stands, your question is too broad.

I'd also question whether it's really related to world-building... but I suppose it could be.

Congratulations! You just put container ships out of order

For your last point, you seem to be misunderstanding the portal. If you move something halfway through the portal and then stop, the item will not be moving. Part of it will not detach because the item has no speed. If it pokes through the other end of the portal and both people push on it, it will move like a normal object. To test this, punch a hole through a piece of cardboard and stick a pencil through. You can't see the part in the middle of the pencil covered by the cardboard, but it is still there. What if you push on both sides? If you push equally on both sides, the pencil won't move

Two questions: 1) Is mass a factor, either in power consumption, or ability to use the portal? 2) What happens to matter inside the portal if the output portal is offline?

If you ever want to consider people transportation, you are going to have to seriously deal with the "lost in transit" issue. No one will ever send people via your portals if a power outage could cause them to cease existing.

Following up, in order to monetise your invention, what is to stop someone from releasing knockoffs at half the price? Are you offering the portals as a product, or transportation as a service? How do you address the military/legal aspects to prevent the government from confiscating and burying it?

"Testing has shown that people tend to feel a bit uncomfortable if they're travelling in a fast vehicle that goes through a portal where there is a jarring change of light/scenery" If you put both portals in tunnels or buildings or some such, you can easily make it look exactly the same as, or a continuation of, the other end and the problem goes away.

Safety seems to be the benefit. I'd use it for transporting potentially hazardous materials, to avoid another Lac Megantic disaster

While I tend to agree with the desire to close this question, I disagree with the unclear and opinion-based reasons utilized thus far. This question isn't asking how to build the device or the portal system, but rather how a character would use it to amass the most wealth. This makes it story-based, as a character has many options on the road to success and the one chosen is going to match the interests and expertise of the character in question.

Security.. presumably one could build the portals in secure facilities.. military bases etc. Whilst travelling the items are secure. You could transport gold or vip's safely etc. That alone would make it indespensible.

"...anything sent to them while they are offline is lost." Lost where? Nuclear waste disposal never see it again because it's lost in subspace kind of lost?

On the "Lost on no power" issue - is this a "tunnel collapse" lost, or a "can't catch" lost - i.e. if we take the 60 miles at 60 mph example above, do both portals need to be active for an hour, or can you safely turn it off for 40 minutes in the middle, then turn it back on 10 minutes before they are due to arrive?

What happens when you increase the speed of the object while pushing it in? Does the last part of the object arrive before the first part?

@Tero which actually works well with some mythology where a person travels into an "otherworld" like the land of faeries, for example. There is a common pattern of the person walking into a fog, a hole, a cave, a think forest, etc and emerging some place else. This is not dissimilar to your suggestion - the fog/cave/etc could just be masking the jarring effect of the transportation. Since these portals are "slow", this could also explain why time moves differently in the fairy realm. It doesn't, you just walked into a portal and took you hours to get there. Ergo, faeries use this tech :P

"Portals can't be moved while active". Do you mean they will turn themselves off if they start moving, or that they are physically unable to be moved? In the latter case, what would happen if you plug one in on a high speed train? Would it just stop dead relative to earth and be destroyed by the moving train? Or would the train be destroyed by suddenly hitting a stationary object inside it?

"Can't be moved", or "can't be accelerated"? Big difference. If it's the former, relative to what can it not be moved?

@DavidK I would simply have a multitude of generators create the energy, and have some large backup batteries doing the rest. A single country already works on multiple power stations to provide power, if one goes off the grid for some reason assuming enough is produced by the other stations the energy remains on. So if you make sure that you have for example 6 generators/batteries and you need at minimum 2 generators to be online, then even if 4 of them fail you will remain safe. This would mean that you make sure there's always a few backup systems not connected to the powergrid online.

What happens if you put in a fast object after a slow one? Do they collide in the pipes and arrive in order, or do they have arrive out of order and potentially fuse at arrival?

The former results in EM bursts as each arriving object releases all of the photons it has been carrying though the portal. The latter means that any transfer gets merged with a good amount of air that entered the potral at all sorts of moments both after and before the object that is being sent.

A lot of comments, a lengthy description in the Q, and on hold as unclear. Even if this question can be made entirely clear, it's POB and shouldn't be reopened. Please remember that SE's Q&A model is one-specific-question/one-best answer. Your question is not specific. Worse, it's very subjective. Pure idea geneation Qs are regularly closed. Do you have a specific question? Can you explain the criteria for a best answer?

May I steal this as a plot device? :) I promise to credit you.

I do wonder what happens if one moves the portals instead of the objects. Say one portal is moved over an object with 0 mph velocity, how does that object emerge from the portal's counterpart?

Do the portals need to be aligned?