deskthority

I wonder what is "Please, use this and only this text" part.
Where should I use this text?
In other words, How should I use this "R5 0000 username"?

Hi! I ordered Round 5 keycaps. This is my invoice. --------------------------------------------------------------------------------------------------------- Pay: XX.XX USD \___________ Amount to be paid within 7 days via PayPal in USD (no gift!) To: XXXXXX@gmail.com (mashby) \___________ Send money ...