If it isn't compatible then there is no sense in keeping it, unless you change something else like webhost or shopping cart.
Just an FYI -- There are contracts made under CERTAIN conditions that allow for a 3 day cancellation, but it doesn't apply to all contracts. I don't know if you would qualify or not under these circumstances.You SHOULD be able to get your money back without any grief. Depending on your state, most states give you 72 hours (3 days) to back out of a contract.
The Cooling-Off Rule: When and How to Cancel a SaleThe Cooling-Off Rule: When and How to Cancel a Sale
If you buy something at a store and later change your mind, you may not be able to return the merchandise. But if you buy an item in your home or at a location that is not the seller's permanent place of business, you may have the option. The Federal Trade Commission's (FTC's) Cooling-Off Rule gives you three days to cancel purchases of $25 or more. Under the Cooling-Off Rule, your right to cancel for a full refund extends until midnight of the third business day after the sale.