Hi,
I have experience of creating WHMCS Payment gateways, it takes more effort than you may have imagined, I have created two for different clients in recent past. And it takes a lot of debugging and time/effort. So my bid is $250. Following 3 services will be provided by Gateway.
1. one-time payment (invoice payment)
2. refunds (if supported by your payment processor)
3. Recurring payments (if supported by your payment processor).
Additional Note:
I will provide one month free of cost support, but you will have to release the funds upon completion of testing, and support will be unconditional...
Let's discuss more in details
I have made two payment gateways recently
1. for HyperPay
2. for QuickBooks