There is an entire section about Client payment on our Support desk.
When you reach the payment step, there is an option to switch to post-payment instead of pre-payment. The costs are 3% extra for a 30-days payment term.
Note: this is offered as a full-service, and it is subject to approval from Peerwith Team. In general, we will only proceed with this option when an institution is involved with separate contact details for a research Support office or other Administrative contact.