You can use Royal Mail 1st Class stamps to post letters, large letters, and packets to Guernsey.
Your letters must weigh no more than 100g, be no more than 240mm in length, be no wider than 165mm, and be no thicker than 5mm. Large letters must weigh no more than 750g, be no more than 353mm in length, be no wider than 250mm, and be no thicker than 25mm. Packets must weigh no more than 1kg, be no more than 61cm in length, be no wider than 46cm, and be no thicker than 46cm.
It will cost £0.60 to mail a letter. Large letter prices: up to 100g is 90p, 101-250g is £1.20, 251-500g is £1.60, and 501-750g is £2.30. Packet prices: up to 750g is £2.70, 751-1000g is £4.30, 1001-1250g is £5.60, 1251-1500g is £6.50, 1501-1750g is £7.40, 1751-2000g is £8.30, and 2001-4000g is £10.30.
Customers posting items to Guernsey must declare to customs anything other than correspondence.
Of interest, Channel Island residents sending gifts through the post to the UK mainland now have to pay VAT if those gifts are worth more than £36. If the correct VAT has not been paid on an item, the parcel’s recipient must pay the sales tax and a £8 Royal Mail handling charge.