When I sell a gift certificate, I write the dollar amount that the certificate is for - ex. instead of 'your gift is a one hour massage' I enter 'your gift is for $60.00 toward any services offered'. On the inside of the cert (it folds in half & opens to 11.5" x 5") I list all the modalities & durations (30 min/60min/90min, etc...) that are available. I also have space on my cert for gc policies... cancellation/redemption info, etc.By listing the amt of the gift, not only does it let the recipient use the cert for any modality they want, but I also state that after one year from the issue date/date of purchase, that any pricing difference is up to the recipient, again, this is spelled out in my gc policy printed right on the cert. Since I live in a state where exp. dates are illegal, this gives me the ability to charge more if someone redeens a cert three years from now & prices have gone up.Back to the original Q - If someone is interested in upgrading to a longer session/different modality, I charge them the difference i.e. If the cert is for $60 and they want a $90 service, they simply pay the difference of $30.