Selling $20 credit x up to 6 month, might be longer as I've got the card in December 2021 and I believe I should keep it open at least till January 2023.
Not sure how others do it but for simplicity, I think I am going to ask for one time upfront payment for how many months you want at $15 per month and I promise I will maintain the card during that time. (I have to maintain it or I will get 150k MR points clawed back I am sure)
On the other hand, I will have another $20 credit x 12+ month coming up soon as I just got my wife to apply one today and got approved. I would think by next week I shall have the card in hand. same $15 per month.