Modifying the site to support this is almost as much effort as just building a new site at this point. Some options
1) they can pay by credit card and pay the balance off with them
2) you can manually change their balance for each payment. This takes alot of coordination and is probably going to be a headache.
3) you can go in and change the partial payments so they push out payment another X months/years and payments are now low enough most can probably pay them. This is easiest but risky and everyone has the same schedule so it may not work 100%.
4) B&H Photo created a credit card for themselves. They made a deal with synchrony bank to take the sales tax out of the interest so that the bank gets less interest, BH still pays sales tax, and the customer doesn't pay sales tax. I have it and it works. I use them for nearly everything they can support now. The card is through this bank
https://www.synchron...oter_syfbiz_int
5) There is always Klarna for financing
https://www.klarna.com/us/business/
The last two will probably take some development effort and you are back to the problem of modifying code but you can check to see. Just changing the payment schedule is the easiest and fastest way. $100 a month for whatever time it takes to pay the balance should do it. The balance would still show in My Trips so if you change it again later and it won't mess up their records so they can just keep making payments. It is risky that they won't pay after the trip. You would probably want first time trip goers to pay in full or they may not pay after the trip is over. Repeat trip goers are probably not a problem.