Webflow + Memberstack + 3rd party billing?

This site is still in the idea phase, so there is no link to share just yet, but I have a question. But first, here’s the need:

A website for a Yoga instructor, with a gated content area for their courses. These courses are of 2 types: some are online, and some are live in studio sessions. I want to make it so that their clients can book appointments for the in-studio sessions and buy the online courses through the website.

The clients must have a customer dashboard where they can see their online courses (nice to have would be progress on the online course). Also nice to have would be for them to see their sessions, if they booked them.

During the checkout process or before they can attend their online courses or in-studio sessions the clients must review and sign a contract (probably a PDF).

For the booking of the in-studio sessions, we need a calendar to be displayed on the site, where people can see the sessions and be able to join them. It would also be nice to have it on the customer dashboard.

Solution + questions:

I found this Premium Course Template | Webflow Template and I confirmed with Memberstack that this is a great starting point for what I need.

Calendly is one of the ideas proposed for the calendar, so I also have that covered.

I’ll also use a Webflow theme to get the project going fast, so everything up to this point is set.

Where I have a big problem now is trying to integrate the checkout system (Memberstack) with a billing system through the Memberstack API so that when a customer buys either a course or books a session they can automatically generate a bill that gets sent to the client. Also, the billing system is from a Romanian company, I don’t know if that complicates things.

What I know from Memberstack is that it needs custom development, but it’s achievable.

My question for you is, could any of you help me do this? If yes, how much would you charge for just this API integration?

Thanks so much for your help!