What if the Bitcoin fees alone spike beyond 0.0002 BTC for quick transaction confirmations? Won't you be operating in losses?
IMO it's not that difficult to find the solution for things like that. Either change the fee or use 1 sats/vbyte for every tx. I don't think they promise instant confirmation, just the 1-3 target (which could take hours or days if the mempool is busy). Can't say the customers will be happy with that though. If anything, I'd be more concerned about the strength of the encryption.
Our fowarding service with flat rate fee
https://developer.bitaps.com/forwarding, successfully works several years. We use payment agregation to make payout transaction.
Example
https://bitaps.com/10282766f66076cd357ea8e437c819bf3d66b632b7ab62a7488756fec4e324e4You can see in this transaction that we agregate 10 payment and make 1 payout transaction. It works well enough to maintain flate rate fee.