In this case, you might somehow use ``.
* `` focuses on the merchandise being returned from the customer to the shop.
* `` focuses on the money being returned from the shop to the customer.
Often, two things happen at the same time, but depending on what kind thing was sold, it is more appropriate to focus on one or the other. In addition, for things like train tickets, they are not reused by another customer; they will be probably just become void, and/or thrown out. In that case there is no process that will be described as ``, so it is better to use ``.
In Japan, customers are generally considered to be in a socially higher status than the shop clerk (of course only in that temporaral situation), so you don't have to be too polite, but may use the polite form. But note that in Japan, returning things is not done as frequent as in U.S. For things like tickets, there should be a clear policy somewhere regarding refunding.