I thought it worked like this:
You can only get paysafe vouchers (I only ever use PS and not my cards) vouchers in increments of £10, £25. So if my advertising choices for VS come to, say, (depending on which upgrades you tick) £12, you use two £10 PS vouchers and you lose £8.
When I've done this, I don't see anywhere that says I still have £8 to spend?