More and more shops are doing this - I too always decline because I am not sure how it will work if I need to return the item as I don't keep all my emails on my phone. These companies already have my email address as I also order online from them so I am not bothered about that. In fact, the marketing side of things didn't occur to me. I do think, in the longer term, e receipts will become the norm. Is it not more to do with cutting down on paper, like e bank and financial statements are more the norm now and you have to opt in for paper ones?
If asked in a shop for my email address, I say I don't have one. So far as I'm aware, it isn't yet compulsory! I don't give my phone number, either, unless there's a good reason it might be needed, e.g.: to arrange delivery, etc.
I made the mistake many years ago once of giving my email address to a local hairdressers. I was deluged with rubbish from them and the only time it eventually stopped was when they closed for good.
Nearly everywhere I buy from lately in shops ask for my email address to 'email receipt to me'! I realise this is for marketing purposes.
I find this really irritating as feel it is an imposition and annoying having to unsubscribe constantly when emails arrive.
Argos yesterday - just said no thanks till receipt is fine. Took item back to Screwfix, reimbursed onto my card, no details asked for then went to buy a £2 item and asked for my name and postcode. When I asked why said in case wanted to return it!
I just fine the whole practice underhanded. Does anyone else feel the same? Or am I turning into a grumpy old woman! Sorry for the moan.