Determining amounts to be paid on invoices

Determine the amount to be paid in full settlement of each of the following invoices, assuming that credit for returns and allowances was received prior to payment and that all invoices were paid within the discount period:

$\backslash$table$[[,\backslash$table$[[$Merchandise$],[$Invoice Amount$]],\backslash$table$[[$Freight$],[$Paid by Seller$]],,\backslash$table$[[$Customer Returns$],[$and Allowances$]]],[$a$.,$$$18,000,-,$FOB destination, $\frac{n}{30},$$$900$