how to return the variables between forms?

the first form is an invoice form, which will call the payment form, then print the invoice.

in the invoice form, I did:

do form payment with total_invoice

then I need to return the values of payment: 'cash, creditcard, cardnumber, cardholder, debitcard' back to invoice form.

how can I do that? I don't want to use public variables.

regards,

Jerry