Sponges are not an item that is available in vanilla minecraft survival.. Since it is a creative item only, I see no problem making it available only by donation. Since you must have the donation to use the item, it seems silly to make it a shop item. Heck, why stop with sponges? Make lava buckets and flint & steel and water buckets purchasable. This way we can have lots of angry builders who "accidentally" bought something they can't use. If you want to buy some, buy from people. There are plenty of us out there who have stacks upon stacks of these things. This is just my opinion. If you can get it added, great. I just don't think it is needed.
If you read the whole suggestion, you would know that I said: If someone doesn't read a sign that is right next to/above the buying sign, that will most likely be bright colors, it shouldn't be the server's problem.