What are my rights - went into shop, paid, but turns out I was mis sold the items?

I went into a shop the other day with £350, spent the full amount on goods - paid via contactless app (iPhone).

Turns out however that the payment was declined, but the cashier wasn't aware at the time and sent me on my way. A week has passed and a letter has been sent to my address stating that I haven't paid (this is how I found out)

What are my rights to the goods? Obviously a good man will pay but is this their fault thus, without my resolution, they're out of pocket?

It was YOUR responsibility to pay (not "attempt" to pay) before you walked out of the store with the goods. Your receipt would clearly state whether the payment was accepted or declined. Unless you pay, you can be charged with theft.

"but is this their fault" Partially theirs and partially yours. They have attempted to correct THEIR error, now it's your turn. Refuse and expect the police, a process server or a collection agency to come knocking.

In the US you would be arrested for stealing if it didn't go through. However, this would never happen here since technology would catch it without the cashier letting you just walk out. You weren't mis-sold items… You planned to buy & pay for them but it is up to you if something isn't charged correctly to pay for them.

Keeping the goods would be considered unjust enrichment, so you must either pay them or return it. Who's fault it is isn't relevant. If they sue you, they will win.

They are OUT OF POCKET ALREADY on the fees the card issuer charges for decline, and they have NO MONEY for the merchandise, which you ADMIT is not paid for. The goods are again on the OPEN MARKET and whoever shows up and pays, can buy them. YOU should also pay them the fees they had to pay for YOUR CARD being declined.

You have no rights and if you had looked at the receipt you would have seen it was declined.
You either pay up or return them. Keeping them is theft.
UK

You have no rights. The sale was made with the expectation that the seller would be paid. That did not happen.

You can be sued for any lessened value of "the goods."

There was no "mis selling," whatever "mis selling" means. This could turn into a charge of criminal fraud against you.

I do not know why you think it is someone else's fault. You agreed to pay and you have not paid. You would also be liable if you check were returned for insufficient funds, or if you had paid with counterfeit bills.

They're not out of pocket, you stole something. You know the payment was declined, and still kept the item. You have NO rights to the goods.

You have no rights to anything if you haven't paid.