It's also easy to overlook the euro issue when the majority of popular hosting providers charge in dollars - and I can already see people making this mistake in the thread.
340 million people live in a Eurozone country.
320 million people live in the USA, plus there's Ecuador and El Salvador.
(516 million people live within the Single Euro Payments Area, where international bank transfers in Euros must cost no more than a national bank transfer. My employer in Denmark has a Euro-denominated bank account; this is presumably typical for businesses in the non-Euro countries in Europe.)
You mean that you can't prepay when you think currency exchange would benefit you? Well, if you think you're in a currency exchange sweet spot, go ahead and exchange. Then just leave the money in your bank account for next month's bills.
What's that? Your bank won't let you hold a balance in another currency? That sounds like a problem with you bank, not you hosting service.
My €3/mo was (IIRC) more like €5/mo once conversion fees were taken into account. This €2 flat fee would not have been a problem had I been paying for 12 months at a time, but Scaleway do not offer this, in contrast to many other hosting providers on the market. After inquiring, I was told there was no current plan to change this policy.