Ok I get it now. Cool concept, and I may make some pass-line bets to help you test stuff out, but I'm gonna mostly hold off until you allow placing odds bets and place bets
I have a couple ideas, and you can take with a grain of salt, but I'm only offering them up because I was considering doing craps as well, and you beat me to it, so maybe you can use these ideas! **Please note I mean absolutely no disrespect at all with these suggestions, nor am I suggesting that you hadn't thought of them yet... but if you hadn't, I think it could work...**
In your current setup, the bets I'm talking about could be possible in the following scenarios:
Pass-line with odds. Let's say you allow 2x odds that pay true odds (since that's really what that bet is), you could maybe allow a person to submit .3 btc, one of which is the .1 passline, and then if the rolls establish a number, the .2 become the odds until win or loss. If they crap out or get a 7, you could refund the additional .2 btc along with their winnings (or just the .2 if they crapped out). Otherwise they'd get back the .3 plus winnings. I understand transaction fee, but odds bets are the only true odds, so I feel like those should be transaction-free
It's your call of course.
Also, I think place bets would be possible in your scenario (or place bets plus pass-line odds) by letting the player specify their pass-line bet (and any odds), and then what they want to place (with the understanding that they can't remove place bets since you're limited... that could help the house advantage and limit your transaction fees and maybe help paying closer to traditional odds). So anyway, if I was playing .1 pass with .2 odds, then maybe I could lay down 2.2 inside, submit 2.5 total, and then you let your roll magic work and then go line-by-line noting each of the place bets and what they would pay, and then getting to a 7 or whatever was established on the come-out roll, and then refunding if necessary (due to like a 7 on first roll or something). It could be quick, or could go on for a couple rolls.
Anyway, I mean no disrespect at all, I just love craps and offering up the ideas!