No, you do NOT have the right to do that: doing so would be, depending on the exact facts, one or more of breaking and entering, burglary, theft, vandalism (if you did damage in the process), etc. Your recourse is to sue them for the money. You would sue for breach of contract--violating their obligation to pay when you fulfilled your obligation of delivering product. You would also base the case on unjust enrichment--on the manifest or obvious unfairness of allowing them to keep their money and the product both. For amounts under the limit for your small claims court, you are most likely best off suing in small claims "pro se" (as your own attorney) to keep costs low; for larger amounts, you should retain an attorney.