You won't be able to post dishwasher via PO.

Neither can you post powerchair or sofa.

You will need to do collection only, so revise listings before you get more annoyed buyers.

You have on your listing that you'll post it to them for £5.88, which is what they've now paid.

You should have made that a collection in person listing and, as advised by @tobiasd4  you need to revise the postage section of your other listings.

You may need to cancel this order, you'll need to contact your buyer and explain that you made an error with the listing. They may not be best pleased though.