It appears to be a one time use code so you could say buy $30 of credit on stacksocial and redeem that for $30 of paygo credit in one transaction. $5.99 for $30 of credit is excellent. Or buy a month of whatever plan you want.
I signed up a line, and it worked fine. I didn't port a number, though. By the end of filling out boxes & clicking, I was literally logged in to the new account.
I also got the $1.50 charge, which seemed odd because it didn't appear on the invoice...... I thought it might have been taxes & fees based on applying the coupon to PayGo credit, but that line is pretty clearly indicated as $0, and a $0 total.
Zubrin, if it hasn't worked out yet, you should reach out to Tello. The coupon code should make the event trackable. Either they have the code applied, or try starting over and applying the coupon code again.