EDIT: DEAL OVER.
There's a lot of information in this (long)
slickdeals thread. The pertinent info is on page 1.
The better coupon to use ($60 off the $80 price) is VOLTGOODSHOP60.
I used a cash back site (ebates) that offers $17.50 cash back, and it went through already, making this essentially a $3.89 phone. Some others are using Be Frugal and maybe other cashback sites, which offer $35 cash back, except in their terms it says you don't get the cash back if it's more than the cost of the phone. Yet some report that they've done this before on other phones and it worked.
Note that the cashback sites show you a button to clip the lesser-valued ($55 off) coupon code VOLT5CJ55. Instead of clipping that, I used the $60-off VOLTGOODSHOP60 coupon at checkout and ebates didn't care.
Virgin Mobile is a Sprint MVNO.
Confirmation e-mails show the full $80 price, but people are reporting that it's just an error in the e-mails. People that paid via PayPal went and verified that they were only charged the $20 + tax. If you're still concerned, you can print or save the final receipt as proof.