Currently, the Deposit Paypal plugin is setup by default to accept deposits in the service module if Staff section so that a fixed percent or fixed amount per service is charged.
However it us much nicer to have an option, that customer pay per appointment a fixed amount.
The reason for this is that for example if I have Chain (Add-on) appointment, and client adds 2 or 3 services, and if I have set it to charge £2/service as an example (as per current default for deposit plugin for each service) then for each service it will charge £2 and if client chose 3 services then she will pay a deposit of £2x3=£6. Another client booked 2 services for her appointment so she will pay £4 deposit on same basis.
So problem is to keep a record of this as to who paid £2 or who paid £4 or £6 and so on.
Hence, the option to have a fixed amount deposit PER appointment as an option is suitable under such circumstances and especially when you have a Chain (Add-on) plugin installed.