I like the "upgrade" feature. The popup window is clean and simple for users. However, when a user clicks to upgrade their account, they are taken directly to the payment page rather than the shopping cart or the signup page. This is a bit of an issue for me as coupon codes can't be applied to this page. When upgrading a product using the "product upgrade paths," how can coupons be applied?
Unfortunately coupon can not be applied in event of upgrade. You can submit feature request in our bug tracker http://bt.amember.com/
Yes, we already implemented this feature and it is included to latest version. You can enable it at aMember CP -> Configuration -> Setup/Configuration -> Advanced (allow usage of coupons for upgrade path)