Payed is the past tense of the word pay which means to seal a deck or hull of a ship with tar so as to prevent leakage. However, its use remains limited as it is known as a nautical term having to do with ships and ropes.
"The ship looks magnificent but it has yet to be payed."
Paid is used in any general sense apart from the nautical class. It involves the transferring and giving out of money. It is commonly used as a financial term.
"He paid only half of his debt."