He or she is running a phishing scam, asking you that once you receive their Cheque, to cash it and forward the excess to a shipping company. The cheque will be bogus and you will be ripped off for the total amount.
This is NOT the first scam I've encountered over music gear purchases.
Anytime a buyer offers to send a cheque, and their story includes a school, or church or similar organization BEWARE.
Cash only, Always.