Firstly, good and services that are consumed/used while you are here are excluded. E.g. hotel stays, food, etc. (Although, certainly under UK rules, items don't need to remain boxed, just clearly look new, etc).
Also certain specific items (generally cars, boats, etc) are excluded under the scheme, as separate dedicated provisions exist for them. Also restrictions on some items to be used for commercial purposes, or where believe exported for subsequent resale, etc.

However, retailers themselves are free to choose what items they sell they want to offer VAT reclaim for under the scheme.

They are also free to charge whatever administration fee they like for the service, in some cases it will be flat fee, others percentage based - however almost every store will have a minimum (or certainly a common sense minimum, don't bother trying to do it for goods under £50 etc!)

However, many stores will let you combine multiple recipts from the same store (on the same day) into one reclaim form - with one charge. And if you are reclaimin for an expensive item then they will usually let you happily have loads of cheap ones on the bill, or a large number of cheap items giving a big total.

Always ask first: Do you operate/offer the VAT Retail Export Scheme (for visitors to the EU to reclaim their VAT on exit), - and how much do you charge as an admin fee?

- The second question is good for reasons, one is that it ensures the answer to the first question is accurate (indeed if the first question answer was 'No', ask them to check or ask someone else, espeically if you have reason to believe they do - listed on a website etc, many inexperience sales staff don't know about it).

It is also important to know the fee as it can make a big difference. If a small total value from that shop, % based normally is to your benefit, if a large total value flat fee wins.
I've bought thousand pound computers in the past from one shop becuase flat fee, and a hundred pound camera items from another because % based and slightly cheaper after the reclaim.
[Obviously you also need to factor in the actual price they are charging for the item, and the non-certainty of reclaims (they can go wrong, or be forgotten about when leaving.]

You need to show the reclaim forms to the customs officers at the point of departure from the EU fiscal (VAT) area.

So, in the case of items travelling in checked in luggage, take your case to customs before you checkin to get the form stamped. Even if the luggage will be through checked via another airport. If however, your luggage will not be through check do it at the last airport you will check it in at before leaving the area.

For goods carried in hand luggage, show the forms to the customs officer at the last airport you pass through leaving the area.

E.g. if you are visting London then going to Paris for a week, you will not be able to get your forms stampped upon leaving the UK as you are not leaving the EU fiscal (VAT) area at this time. But the reclaim forms from the UK stores you will be able to get stamped alongside thouse from the paris stores when leaving later.

Assuming you are flying direct CDG > airport outside the area, then yes show at CDG.

If you are flying CDG > connecting at an airport inside the area > then airport outside area, you should show forms relating to items in your checked luggage at CDG (assuming the baggage is through checked), but items on your person/hand luggage at the connecting airport.

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? :D

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).