I had all sorts of pains with portal tracking until I did all of the following (slightly Chrome-specific but same principles apply to others):
1. Always go through the portal and make the purchase in a new incognito window.
2. Don't allow any adblocking extensions in incognito.
3. Allow all cookies.
Ever since doing the above, all tracking has gone smoothly. Some of the above might not be strictly necessary, but I got tired of taking a shot in the dark every time I used a portal.