I believe Amazon increases the price with time, for a given customer. Now I can’t find anything cheap on Amazon, all the crap seems to be at +15€ compared to shops.

It would be interesting to make a new account to test that. I assumed it's because they have so many millions of customers locked in with free ("free") prime shipping that they know they'll be fine price matching upwards as long as it's a similar price elsewhere online, because they are who most people I know think of when you "need something like tomorrow" and don't want to go shopping.

I think a lot of sellers also include the shipping cost in the price of the item and ship it to Amazon so it can be PRIME-eligible.