You can either pay for payment processing fees yourself or you can pass on the fees to your client.
If you want to set up a default for who pays payment processing fees, you can do so on the App Setup page and clicking the ellipsis to the right of the Billing App and clicking Edit:
This will open the Billing settings, where you can enable or disable the Absorb transaction fees option:
On every invoice and subscription you create, you can overwrite the default set above by clicking on Advanced Settings and toggling the Absorb transaction fees option.
For more information on absorbing fees, check out this article: What does it mean to absorb transaction fees?