Will the recipient have to pay

[Changednayme]

I am going to send a small parcel through a local red post box and I have attached 2 first class stamps. Will the recipient have to pay something. It’s In a cardboard box wrapped in a carrier bag that’s sellotaped

Depends on the weight of the parcel - did you weigh it first?

I don’t have weighing scales but it’s light weight

Yes I'd imagine they would have to pay. It doesn't sound like a very small parcel if wrapped in a plastic bag so I doubt 2 first class stamps would be enough for a parcel.

Depends if the stamps cover the amount of postage for the size and weight of the package. If it doesn't they usually put a card through the door to say you need to pay a fee. Examples on royal mail here:
personal.help.royalmail.com/app/answers/detail/a_id/143/~/ive-received-a-grey-fee-to-pay-card

Who’s door do they put the card through because my address isn’t on it

The recipient will have to pay when they go to collect from the sorting office. Why don't you take it to the post office yourself to save your recipient the added fuss?

The card will be out through the recipient's door, usually saying that they have to pay x to get the parcel.

The door of the recipient. Who would then have to pay to get the parcel along with a processing fee.

Is it something the recipient is going to want to go and collect and pay to get?

Through the door of the address on the package. They take online payments so you could pay it for them once they get the card. Obviously you'd need to let them know in advance you had sent them something and to let you know when they get the card.