First @ Tari. Thanks for your comprehensive troubleshooting research. I use Firefox as an external browser, and can assure you it's impossible to return to a previous search context following a purchase. All the browser back button does is return me to the Invoice page, with subsequent hits of the back button only serving to refresh the Invoice page. As Freya noted, use of the "Return to product page" button there, is completely useless, and does nothing more than whack you to the post-purchase "Thank you" blah-blah, Featured Items page. But thanks for the insight on Chrome browser performance.
Second @ Tamara. Your workaround is a procedure I had not ever considered before. And with a little care and feeding of new tabs, does get around the key issue fo protecting your current search context. Thank you. Thank you.
Third @ Freya. What can I say. Succintly and acurately expressed. I completely agree with you on all points, Freya. Simply put, the system cries out for a post-purchase RETURN TO PRODUCT PAGE button that works, meaning it returns you directly to the search context page from which you made the item purchase.