I think the vouchers for money off are tied into the accounts they are sent to i.e - non-transferable. I do know with Ocado that they send out vouchers to customers who don't spend a great deal with them to encourage them to spend more.
I know someone who receives them after about 2/3 weeks of not ordering from them. It's usually a voucher for 15% off if she then places an order for delivery on a Monday/Tuesday for some reason. She's sent me the code before now which is how I know they're account specific after I tried to use it and it wouldn't let me. Made me a bit for some strange reason. Felt like I'd been told off!
Yes that is correct if you don't order for a week or so they start to email you various offers etc. I actually registered with a different email address to get the £20 off but the driver recognised me and asked about it. I just said I couldnt remember my old password so had re-registered, but don't think I will do it again in case I get rumbled!