The best thing to do is to work out the actual cost of making the card, and then add on the amount per hour that you want to be paid for making the card.

E.g. if a card takes 10 mins to make and you want to be paid £10 per hour - then your time would be £10 divided by 60 mins x 10 mins = £1.60 (approx) per card PLUS the actual amount of stash used.

I would keep accounts in Excel of incomings and outgoings including petrol/time to deliver (based on £10 per hour), going to post office etc. etc....