UPDATE Table1 SET Field2=Table1.Field2-1 WHERE Field1=Value
Where Field1 = unique value to identify record and Field2 = the value you want to decrement by 1
As for creating an invoice there are several ways. Datareport, Crystal Report, or one of your own design via the Printer object. But to be able to do this you will need a purchase table to record the items purchased with some unique ID that ID's the customer so when you go to print the reciept, the table can be queried against.
Good Luck
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