You asked:
Quote: was wondering what the difference between "money" and "monies" is.
Also, when writing a business letter, do you write: seven dollars, 7 dollars, or $7.00?
In most cases "money" is its own plural. When several sources of money come together, "monies" can be used. For example:
The monies from the bake sale and the and candy sale were combined.
This usage is correct--but it is a bit old-fashioned.
The answer to your second question depends on the structure of the sentence:
Seven dollars will be needed to pay for shipping.
Shipping wil cost $7.00.
7 dollars would not be used by a native English speaker--unless that native English speaker was to lazy to write s-e-v-e-n.