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:
[email protected] (mashby)
\___________ Send ...