Handling Charges, and your forwarding options, are based upon the Royal Mail's size and weight definitions below.
Something to consider: Although classed as a 'Letter' for the purpose of calculating our handling charge on delivery, this same letter may incur 'Large Letter' Royal Mail postage when forwarded as a single item, simply because the tamper-proof postal bag in which it is enclosed adds overall size. This is rarely important of course, because we almost always forward several letters in the same bag!
a letter is. . . a letter!
To be classed as a Letter, an item must be under 100gm in weight, no larger than 24cm x 16.5cm and less than 5mm in thickness.
when is it a large letter?
The next step up, with a maximum size of 35.3cm x 25cm and up to 2.5cm, (around 1 inch), thick is the Large Letter format.
A Large Letter can weigh as much as 750gm and might contain things like A4 documents, cased DVDs or magazines.
. . .or a parcel?
The Small Parcel format, weighing up to 2Kg and with a maximum size of 45cm x 35cm x 16cm, can be carried by Royal Mail 1st Class post to addresses within the UK and abroad, just like the two letter formats.
You can, however, use your mailbox for items up to 20Kg in weight and as large as 61cm x 46cm x 46cm (the Royal Mail's Medium Parcel limits) at no extra charge, but must be forwarded by Private Courier to destinations abroad.