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If you ordered on Sunday, the PAYG SIM would not have been picked and packed until the next working day on Monday and depending on Royal Mail postal situation will take 3 to 6 working days to arrive.
You will probably find the SIM will be delivered before the end of the week, if it is not received start a WEB chat and the advisor will be able to check the status.
@Cemrediff you can't cancel the order it's already been picked and despatched. When you receive the SIM there should be return information in the packaging.