Stripe - Can I collect a flat fee in addition to % fee on subscription item

Using stripe’s API I can take a % fee from subscriptions I set up. But want to be able to charge a flat fee on top of the % fee.

For example:

2.5% +$0.30 for every subscription transaction.

I’ve been through their documentation but can’t seem to find a way to charge the fee in addition to the %. I can set up a pending invoice to add a fee, but this adds it on top of the total, rather than taking it from the original amount paid.

Anyone implemented something similar?