I don't want to pay upfront, but afterwards, how can we solve this?

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.