When wallet sends the mint an invoice to pay, it reserves and passes to the mint more then actual invoice amount to account for fees. If your wallet has no exact denominations it can pass way more actually. Wallet does not inspect if it's an invoice issued by minibits mint, but it's actually an interesting point to investigate and possibly cut some more friction (but might require change on the mint side) As you've seen, after payment settles, unspent fee reserve is returned as a change. Thank you for headsup in any case!