Any residual are easy to spend on SK vouchers if you travel SK regularly (so you will have a chance before the voucher expires). You can combine a voucher and a credit card, and it is easy to do on the website. Also you can easily use a part of a voucher, or to combine two (not more) vouchers on a single trip. The last year was so crazy, that I think I used vouchers more than my credit card on the SK website.

The vouchers that SK gives out seem to change conditions all the time, so read the fine print. I never got a voucher with a premium on value (like people reporting above), but recent vouchers seem to state that the residual value can be paid out in cash when the vouchers expire.