You can swap ecash for close to anything, as long as the mint implements it. The only thing that really matters is finality of the swap medium. Because once the ecash is issued, the mint doesn't know what the ecash looks like (due to the blind signing). So the mint cannot invalidate it in case the user refunds the transaction.

So a credit card, which has charge backs and finality in the order of weeks, would not work well.

Physical cash, which has instant finality works well.