The price of a subscription plan can be free or of an amount fixed. Could be possible that the price be up to the subscriber. For example, for an association, there is several plans depending of support to the association. And a sponsor could pay the amount that he want, for example from 300 $.
What di you think about that.
It seems that it doesn't work because when the plan of subscription is free, the payment is not activated.
So, I left a fixed price which corresponds to a minimum and I created a textbox fee field for a donation that the costumer can pay as he wants.
Normally the two field must be totalised, but I think that they aren't.