Yes, because renting is paying for. It's not free? So the shipper has to 1) pay a megafreighter to ship their goods to the highport, then pay to store goods in the highport until they can be put on a cargo shuttle to the surface, then pay the shuttle to the downport. This has to compare favorably to shipping the goods on a smaller freighter that can go straight to the downport.
IF the megafreighter costs enough less *to the shipper* than the streamlined freighter direct to the downport, this will work. That is not how trade rules work in Traveller, but those don't actually make any sense anyway.

So the GM can make up a situation where that's true. But they can also make up a situation where that is not true. It will not be the same from system to system, because the relative costs of orbital vs ground based infrastructure will vary considerably based on what the planet is like.
Regarding Regina, I guess you just have a different map than I do. Regina has *two* class B starports within Jump 4 of it (not in the Regina subsector, btw

). The coreward "top" of the subsector does have a decent number of A & B starports, but except for Efate, Alell (an amber zone), and Feri, they are all low population.
Uakye and Boughene are less populated than Luxembourg. Pixie might conceivably have fewer residents than a megafreighter has passengers & crew. Kinorb is about the population of Wisconsin. Yres and Heya actually have pops approaching the USA. A large jump 3 vessel operating from Efate probably has enough bilateral trade partners to be feasible. There's no practical way for Regina to do large freighter trade with those coreward worlds as there is a wasteland of D & E ports between them. If you decide that Roup's actually got a substantial high port despite being class C, an Amber Zone, and Tech 7, it is a bit more feasible.
The megafreighter thing assumes a level of infrastructure that the Traveller rules do not. A Class B starport per the rules only has an 8+ on 2d6 of having a significant high port. A class C is a 10+. You can (and maybe even should) ignore that. The fiction descriptions of Charted Space suggest a lot of space trade, but the mapped world data doesn't align with that. Personally, I'd fix the infrastructure to match the supposed high level of trade and/or use Jump Ferries to move non jump freighters around. But that's just what I would do. I am not under the impression that it is the one true way to do it.
Again, you can decide that interstellar shipping is so cost effective that even Planet Luxembourg is getting megafreighters' worth of cargo regularly.
You can decide that class C starports are equipped to handle megafreighters. Do what's fun for your game, obviously.