If they had not paid you at all, you would be on good grounds to refuse to follow through on the other two jobs, inasumch as that would be a material breach of any agreement. If they simply pay late (roughly 3 weeks late), it's not quite as clear that their breach would be material enough. The issue then may come down to do you receive the check that was allegedly sent. If it was sent on 11/30, you probably should already have it; if not, you should certainly it by Monday. If you do not, you may be on good grounds to not do the additional work since you have not been paid for the work already done. After all, if they are this late and they want to make sure you do the work--given your communication with them--they could have fedex'd or otherwise overnighted payment. You could also take the tack of asking for payment for the new jobs in advance, given the delays.