We can support adding fees on top of a transaction - but only if the underlying gateway also supports it.
For example, with Stripe, we support adding fees on a transaction through the <
application_fee> tag as seen here in our documentation.
Braintree allows this with the
service_fee_amount field as seen here.
Digitzs also allows split payments - instructions for setting them up are in our Digitzs guide.
If you know a gateway that is supported by Spreedly also supports adding fees on top of transactions, you can send the related documentation our way, and we'll let you know if their approach is compatible with our API.