You are paid in cash at the field by the teams. With very few exceptions, USSF uses the three referee system (one referee + two assistant referees).
View a pay schedule for CASL, MMYSL, MMMSL,
and LAWSHigh School matches
High schools pay by check, usually presented to the official at the field, but sometimes sent to their homes a few days later.
Through spring 2011, the fees are a flat fee of $35 for JV and $45 for varsity.