Hi, Johann!
Sorry, but we cannot provide exact solutions on this matter as it is beyond our support services. We can only show how our product works or help with occurring errors to be fixed. I can understand your request, but it is a custom implementation based on your app that you are trying to build with its Bubble logic behind. The support that we provide with templates covers only general guidance and bug fixing if something is wrong with the template. We cannot provide step by step instructions of every modification that you want to do with the template. However, if you’d like us to set the stripe payment according to new date modification you can hire us.
We offer customization services based on our templates and from scratch as well.
We’re usually working on projects in the range of 8-12k USD to make sure we can deliver something both, us and our customers can be proud of 
Read more about how this works: https://dev.zeroqode.com
Some work from our portfolio: https://dev.zeroqode.com/portfolio?ref=intercom
Our customers about working with us on https://dev.zeroqode.com/?scroll=reviews and https://www.trustpilot.com/review/zeroqode.com
To get you the effort estimates we need to have the requirements about the project shared first.
If all of that looks good - please Send us a Request and one of our managers will get back to you. https://dev.zeroqode.com/?scroll=getstarted
I can suggest you our minimal Support Package 3,750 USD which is equivalent with 50 dev hours (including Project Management and Testing), that can be used on detailed analysis and breakdown of the optimization options/ improvements, on the features you’d like to add, bugs cleansing, and then anything else you may have been planning to do in short or long term perspective, as this amount will be on your balance. Let me explain how it works- when you require anything to be done by us, your dedicated Project Manager will tell you exactly how many hours will that take, if you validate it then we get into the app and work on the requirement straight away.
Thanks for understanding! 
Best, Julia.