You are right, using google script can finish all the task and will cheapest way.
1. For sign-up, we can create a google script to find out whether a new trip is created in spreadsheet and send mail with bank transfer information. If using credit card, we can use google script call paypay api to send invoice.
2. For another feature, when google scripts detect that user is not appropriate for beginners, we can use google service send a mail to staff member, and staff member can reply the email which will be detect by google scripts, then we can continue the process.
Please choose me!