Quite simply, there is no recurring cost when using SPOD. When an order is placed, only the order value and shipping costs of the items are charged by SPOD. The shipping costs depend on the order value, shipping type, and country.
For more information on how costs are calculated, consult the article How are the product costs calculated? and our Shipping Calculator.
However, it is very likely that the shop system or marketplace you are integrating SPOD with will have fees. These costs will have to be verified with them.