Can I post a parcel through a red post box

[Changednayme]

I want to send a small octopus teddy through it and it could fit in a padded envelope

So long as you weigh it and put the proper postage on it, that will be fine. If you don’t pay sufficient postage, the recipient will be required to pay it. Also, bear in mind that if you hand it across the counter, you’ll get a certificate of posting and some insurance cover.