When I visit the Buy Credit page, the target URL loses my webmaster ID or it has another ID appearing there. Why is that?

The sales will go to your account because we use a cookie-based tracking system. Whether or not there is an ID at all in the URL, it’s still being tracked as your sale. The parameters identifying your Webmaster account and program code are sent to the billing server and this information is read from the cookie values.

Payment cookies are not overwritten until they expire. Revshare cookies last 14 days while PPS cookies last 1 month.