Use the "Dollars" number spinner to choose the dollar amount you were paid to take on this proofreading job. If you're doing free work, you can reduce it to zero. The default value when the "Create a Customer Transaction record" panel loads, is $75.00. But the spinner can be decreased to zero or raised as high as $5,000. I strongly advise against doing any work unless you get some money up front. You might take 50% of your fee up front via PayPal, then request the remainder after you've emailed your client the first half of the completed proofread job. Having said that, most of my business via PayPal since 2013 has involved the entire amount paid up front, that amount varying from $179 to $299 for novels with no especial "hurry up" attached to the job. Of course, you're free to set your prices as you see fit.