For a while, it was popular to change the spelling of ordinary words to make them appear more Latin to increase their stature. Receipt is a victim of the Latinizing craze. When the word came into English from French it had no ‘p’, and no one pronounced it as if it did. Enthusiastic Latinizers later added the ‘p’ on analogy with the Latin receptus. This is also how debt and doubt got their ‘b’s, salmon and solder got their ‘l’s, and indict got its ‘c.’
Most of the words that were Latinized did have some distant
connection, through French, with the ancient Latin words that
dictated their new spellings. However, sometimes a Latin-inspired
letter got stuck into a word that had not come through Latin.
“Island” came from the Old English íglund, and was spelled illond,
ylonde, or ilande until someone picked up the ‘s’ from Latin insula
and stuck it where it had never been meant to be.