I admit that this online shopping can be a nightmare.
I often 'check out' only a few poundsworth of groceries with Tesco in order to secure a slot.
As yet, I haven't forgotten to go back to complete the order, but now that I also do online shopping for my mum (with a different supermarket), it might only be a matter of time before I slip up!
And I find Tesco's site very slow, too.
Mum's orders are from Morrison's.
With them, you do have to have over £40 of goods in your trolley to be able to check out and secure a slot.
I've found the quickest way to do this is to stick two bottles of spirits in there, which I take out again when I do the proper order nearer delivery day (Mum can't decide what she needs till as near that time as possible).
If one day I either forget or am for some reason unable to amend the order, she will be on a boozy liquid diet for the following week! 
On several occasions, I have had problems paying on Morrison's site.
I've never managed to pay using the debit card, so have to do it on the credit card instead, and sometimes it has taken the best part of an hour to get the system even to accept that one.
I'm sure the problems with the credit card have been down to them, as it wasn't even possible to enter details of another card at that moment either.
Why I can't use the credit card is less clear - bank and supermarket blame each other!