I do almost the same thing you do, but we rebuild the ECO mechanisms. Our base cost is $200.00, but because other parts are just worn out, the average unit usually runs about $350.00 with shipping.
When one is ready, I send the customer an invoice explaining all the costs, then tell them they can pay by cash, check, money order (explaining the we prefer U.S. Postal Money orders). We usually receive the payment within 7 days, then we ship the unit. We have probably rebuilt over 150 of these units and no one has ever complained about the payment method, and no one has ever not paid.
It is not a situation where you do not trust people, it is only good business to expect payment before you ship anything.
Author, 1st & 2nd editions of Gas Pump ID book, 3rd edition is now available at www.gaspumpbible.com
Air Meter ID book also available