Here in Germany, cashiers are by law required to accept up to 50 Euro coins. I sometimes pay postage fees with smaller coins, many of them, and they never mind, they even explicitly say it to me: "Not a problem in the least". Anyway, it's a great idea to exploit one of the many "unwritten rules" scenarios for an ad :-)

PS: I remember that a few years ago someone was so angry with the government that he paid his taxes in small coins, a whole truck load of them - and they had to accept it (@RLDavies: probably the same here, government institutions have to accept it).