Assess the queue at the main till, and weigh this against the number of items in your basket (3).
Decide that scanning three items at the Self Service check-out can't possibly take longer than waiting in that queue and, with a frisson of trepidation, make your way to the Self-Service lane.
Remember the last time you used Self Service (when you were charged 5p for a bag to put one Fudge bar in). Select 'I am using my own bags'.
Place your purple spotty shopping bag in the bagging area, as directed, so that the machine can 'verify your bag'.
Become confused when machine cannot verify your bag. Wonder if perhaps the bag is an existential oddity, that appears only to you because you want it to be there.
Decide to ponder this question at a later date.
Push some buttons to try and force the machine to verify your bag. It's purple and spotty, how can it not be verified?
Take a great sigh of relief when the machine appears to give up on verifying your bag, and allows you to continue.
Realise that it's probably charging you 5p for a bright orange Sainsburys turtle-killer. Realise that you no longer care about the turtles.*
Scan first item.
Place item in your bag.
Heave another sigh of relief, and allow yourself to start believing it's going to work this time.
Scan second item.
Cry, as the machine bellows 'unexpected item in bagging area', attracting the attention of everyone in the small, city-centre shop.
Wait for member of staff to come over and press a button.
Place second item in bag.
Scan third and final item.
Place third item in bag. Suppress rising hope that perhaps this will soon be over.
Press 'Finish and Pay'
Count out the required amount (£1.52 in this case).
Put £1 coin in machine.
Put a 50p then a 2p coin into machine.
Wait for machine to recognise the 52p just introduced to machine.
After 5 minutes, accept that machine is not going to recognise your 52p.
Cry some more.
Consider paying an extra 52p just to end the horror of it all, and go home.
Decide that that's what the machine wants and press the Help button
Realise that the queue at the Real Person Check-out is now composed entirely of people that came into the shop after you. And that every one of them is watching you fuck this up as they wait to be served by a Real Person.
Explain your plight to the very helpful and understanding Sainsburys man, who looks at you pityingly.
Wait for Sainsburys man to get the keys for the machine.
Watch as Sainsburys man opens the machine, unlocks the cash box, extracts 52p, locks the cash box, and closes machine.
Inform Sainsburys man that you would pay an extra 52p just to make it stop. (He looks uncomfortable and says 'there's no need for that, miss'.)
Re-introduce the 52p to the machine, successfully. Thank Sainsburys man for his help. ('Can I help you with anything else Miss?')
Muster all the dignity you can, and leave the shop with your head held high, clutching your precious cargo to your chest.
Realise that in the time you've been in there, darkness has fallen, the leaves have fallen from the trees and Christmas is just round the corner. Vow to never ever again attempt to use the Self Service Check-out facility in any shop. Ever.
*This was but a momentary blip - I do care about turtles! Don't buy turtle-killing plastic bags!