Here's what we need:
1) One-time purchase for 6 months access to the site. No renewals, 1 payment up front only.
2) Credit-card payment processing through Stripe or similar.
3) True white label, including payment gateway (that means no PayPal)
4) Plays nice with Divi.
5) Strong clean user experience. No multiple clicks to purchase, no requirement to add to cart then checkout, just enter name, email, password, and payment info, click purchase, and done. Ideally one form that can be put at the bottom of a squeeze page.
The more we can control and style the checkout to match our site theme, the better. Barring that, it needs to look good.
We played with iThemes Exchange and found the functionality good but the user experience clunky and ugly. Too many clicks to purchase and its templates just look so unprofessional (and seem difficult to edit to really make look better). If it could be made pretty and provide a quick clean user experience, it would be a great choice for us. If we could get Exchange to look good and provide the kind of UX we are looking for, it would be an ideal solution, since we use their security and sync plugins already.
We also tried out Memberful, which has a great and simple user experience and looks really professional, except it isn't white label (they put their brand on the checkout pages) and they do not (and will not) support 6 month 1-time payment memberships. Their pricing is also outrageous, in our opinion, especially given their feature set.