@Kefrayba I had the impression that he bought other things with the pile of treasure he brought. if all of it gone to some bottle of potions and a cloak then that shopkeeper is really obligated to give him the freebies with the amount of profit he got
@Dezaki The freebies were because he told the shopkeeper his Earth name. Which tell the shopkeeper that the MC was a summon person like him. So shopkeeper wanted to support the MC since he himself had given up on finding the way home.
@Kshatria Pretty much any armor has some sorts of penalties, or upper limits for stuff like the dex bonus which could lower your AC depending on how much dex you have. I've played a number of characters that were better off stark naked at higher levels than in armor even though they wore stuff like leather or hide armor early on.
The clothes and robes made for monks are often pretty useful in such cases since they might have some nice bonuses, if they don't have super specific restrictions on them either anyway.
@Kshatria
You can forget about armor realism in a series that is heavily based on classic DnD and isn't simply implementing JRPG/MMORPG tropes like most other isekai/fantasy stories. If you're not familiar with DnD, maybe something like Dark Souls would make sense? There's an advantage to being a naked piece of bacon in that game - that's video game logic for you.