Airports generally have their duty-free shopping at departure gates, so that you can buy duty-free goods and immediately depart on an international flight, taking the item out of the country. If you keep the goods in the country where you bought them, you'd be required to pay duty and tax on them (which would require you to declare them at Customs). In fact, some of my fellow passengers ran into this last year when our US-precleared Montreal to Fort Lauderdale flight was cancelled. They bought duty-free goods before clearing US Customs, then boarded our flight, but we were deboarded and had to re-clear Canadian Customs (since technically we had left Canada, by airport operations rules). I don't think they were charged any taxes, because of the circumstances, but they had to declare the goods.
In your case, if you bought goods after clearing Customs in Paris, you're now in France, and you can't buy duty-free goods anymore.
The time to buy those goods would be when you leave - which unfortunately gives you the complication of whether it's in stock, as you mentioned. Perhaps you can contact the duty-free store in the airport by phone to see if it's in stock. Again, you'll need to be departing from Terminal 2E to do shopping there, so if you're departing from Terminal 1, you'll need to use the Terminal 1 shopping.