Creating a booking form with WIX Code

We are tiny tour operator that has a website built partly with a Wix template. Unfortunetely Wix lack´s of a decent booking system for tour operators. They only have one for hotel bookings which can not been used in our case. We are going to use Wix "online webshop" for our clients to book their trip and paid a registration fee. This is due to that Wix online webshops is open for coding which make it possible to customize the fields with WIX Code This is what we need: 1) when client makes a booking a calculation must reduce the total cost (se example below*). This is due to that Swedish legislation did not allowed us to take full payment directly for a holiday trip when client makes the order. *Example: Trip cost: 1000 USD Max payment directly is in this case 300 USD. We need help to code this deduction. 2) The final booking form after making the order shall have different fields form like, Name, Adress, email, phone, gender, etc. 3) We like to offer products related to our trips (like Yoga mat, etc). Those item is supposed to get paid online directly which is a ready module at Wix today. To have the system distinguish from the sales of trips we would like to have a code for the trips, like: trw190220. When the system see the trw then is must understand to deduct as described at point 1. If an item dont have the prefix "trw" then it is normal sales. Looking forward to hear from you