First, did you send the request for reimbursement by certified, return receipt mail? If not, copy all the invoices and proofs of payment (cancelled checks, debit receipts and so forth) and send these to the ex with a cover letter, sent by certified, return receipt mail. Tell him in the cover letter that he has thirty days from the date of the letter to reimburse you as per the child support order. If he does not do this, file your motion for show cause re contempt. Ask for attorneys fees (hire an attorney), and interest (12% simple annual in WA). Don't let his threat of modification scare you. It is not likely the court is going to allow him to file another modification again so soon.