It surprised us too. And it is actually the merchant account provider, Adyen in our case, who ask for this reserve. This money is used as a security in case we would charge eg. 100 customers, receive the money, spend it all on marketing and then notice our product is broken and we have to declare bankruptcy because we do not have any savings left.
As I understand it, the amount can vary greatly depending on the amount of risk they see in your business plan.
