Memberships questions-I'm helping a very small NPO in my area finally use QBDT. They are offering 'memberships' to folks looking for service dog training on a sliding scale. For example, one could 'donate' $500 and receive 12 service dog training sessions. They've tried to word an agreement stating no refunds and the money is a donation not a purchase (I haven't seen the actual agreement).

Should this be booked as a liability and accounted for as training sessions are used, since the membership benefits are specific not open ended? Or is it fine to just ledger as a general donation? If they ever get growing, I expect these 'memberships' to grow and involve higher 'donations' involving more training sessions which could take years to complete or use up. 

Thanks!