Thanks for your detailed reply.

I'm not sure if I should be doing my shopping at department stores, so that all my purchases are collated under one tax refund form from Galeries Lafeyette, or get it done at the different boutique stores across Paris. Any suggestions?

FYI, I plan to fly into CDG, Eurostar to London and back, before flying out of EU from CDG. I assume I can submit all my receipts/custom forms at CDG customs (even from those purchased in London).