What to do about an out-of-state lawsuit?

Question Details: We are NJ based tile, kitchen cabinetry, counter top supplier. This individual purchased kitchen cabinetry and counter tops from us. We billed him at his NY address but the job site and delivery made to his PA house. Since it was out state sale and delivery no sales tax applied. It was all cash deal, when we received the order we collected 50% cash in advance and when we delivered the cabinets, the balance of the cabinets plus the 50% of the counter top was collected. We use third party fabricator installer on countertop work. Upon their installation, we asked our remaining balance of the counter top, $1,239. The guy said that he paid all in full when we delivered the cabinets. When we said otherwise, he acted puzzled and later he apologized, he accepted to pay us in the following week the amount in cash since he has no money in his account and no credit card, and wrote and signed a note to my name that he owes me that amount. The following day, he called and claimed that he has a proof that he paid that amount and sent me the invoice we issued with only his signature on it saying that paid in full. He kept saying that the one that our delivery guy signed but kept it to himself. Firstly, we never take the invoice to the job sites when we make deliveries only the packing list which we did on his delivery as well, that packing list even shows what he paid as a balance with his signature. I sent him a detailed email with all this and gave him one week to pay this balance. No reply came so far. Where do I need to sue him? Here in NJ where we are located or in NY where he was billed or in PA where the material and service delivered? Will that be a small claims court? Once I get a judgement in our favor, how I should proceed to collect the amount?

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