Do you want to 'calculate' on your Smartpone? Then stop already. Charlie is all about old-fashioned methods…
The colours of the surf boards are important! The colour yellow takes you from the 1st to the 2nd surf board. The colour blue from the second to the third etc. Enter all 5 numbers in an old calculator and turn it upside down.
The solution we are looking for is SHOES. Can you see shoes on the postcard?
To find the next letter, look for a heart, not a shoe.