Hi Simone - all the markings are struck on top of each other, so they are hard to read. Is there an origin postmark on the back? It looks like it originated somewhere outside the UK and transited through the UK to the US. If it had originated in the UK, it would have been rated 1 shilling, but your cover appears to be 1/2. The Maltese cross shaped marking is the UK transit, but I cannot read the date. The cover was carried from the UK to Boston on a Cunard Line steamship under the UK-US treaty. The Boston marking is an exchange office marking indicating that the cover was pre-paid at the 24 cent treaty rate (24, Paid, and Br. Pkt. in the marking). The 5 cents marking represents that the UK has to pay 5 cents to the US for the US inland portion of the 24 cent rate (the UK kept 19 cents of the 24 cent rate for the UK inland transit (3 cents) and sea postage (16 cents)).
Maltese Cross is the London originating datestamp denoting postage paid (1 shilling cash) - 6 Jan 1854
"5" is a 5 cents Liverpool Exchange Office accountancy mark - since red, it means "credit" to the US of 5 cents. The 1 shilling (=24 cents) was apportioned with 3 cents for internal UK postage, 16 cents for the transatlantic postage, and 5 cents for the US internal postage. Since it traveled on the Cunard Niagra under contract to the British government, Great Britain retained 19 of the 24 cents (3 + 16) and credited the US with the remaining 5 cents. If the letter was sent postage collect, there would have been a black 19 accountancy, as the US would have collected the 24 cents from the recipient and would have to pay to GB the rest of the fee.
The round postmark is the Boston Exchange Office receiving and accountancy datestamp. It denotes that the letter entered the US mails in Boston, from a British ship (Br Pkt - British Packet) on 25 January, and that the required postage (24 cents for under 1/2 ounce) had been paid by the sender.
The red squiggle at the left is from Liverpool denoting 1 shilling paid (reference Colin Tabeart's Robertson Revisited where it is marking M67 for Liverpool on page 174)