Hello,i would like to know what is the value of South Australian stamp SG12 used orange with the sertificate from BPA?Also letter with stamp from 1806s?

And also what can be the value of the letter sent in 1806s sealed from John Britton with 2 stamps on,blue one says/one pound ten shillings/and have on the middle mini silver stamp and 2 clay seals signed.Im not sure about the state of that letter was sent.If there is someone to help me understand from where come from and is there any value?Thank you in advance.