Very good experience
All in all I had a great experience and would definitely use it again. I had no issues with asking the store to print the right form as Airvat helpfully provide all the information you need in the information tab section. I would note that you should upload the invoices as soon as you have them, pre load your passport info and make sure to validate your forms with plenty of time before travelling just so you aren’t stuck at the Eurostar and haven’t got everything sorted (although mine was pretty instantaneous besides my passport being rejected). Not sure how much more you get back vs doing it in store but it’s very easy and nice to have it all in one place / have people contact you throughout to let you know your money is on the way!