Customize Magento 2 Recurring Payments Module

We are running a product rental site at bogo.co.uk. Customers can rent products online. They pay initially a deposit, the first month rent and postage. Then after 1 month they just pay the weekly rental until the product is returned. To handle the recurring payments we are using Amasty "Subscriptions & Recurring Payments" module. Everything is working well apart from with the postage. The postage should only be charged the first time, but with the module it charges postage on every recurring payment. But the customer already has the product so they should only pay postage the first time. For example, if a customer wants to rent a bicycle, then they should pay £100 fees + £40 postage. Then later they should just pay £10 / week. It all works correctly apart from the module charges £10 + £40 postage per week instead of just £10. So the module needs customizing so that the postage amount is not included in the recurring payment. We have checked the code ourselves and there appears to be no simple way to do it. It needs some advanced coding and investigation of the module to change it.