Tello's July 4 promo, running now through July 6, is double Tello dollars when signing up with a referral. That makes 20 Tello Dollars, for both parties in the referral. Yo can use Tello Dollars to pay for additional months. (After your promo rate period is finished, if you're also taking advantage of a multi-month discount.)
A reminder on any of the Tello multi-month plans: if you change plans, or renew early, you'll lose the balance of the promo-- especially important with the Stack Social promo, since you're prepaying for that one. (You'd just lose future discounts with the Tello website offer.)
How do you use the Tello dollars to pay for a monthly plan?
It's a little awkward. You have to renew manually, before your autopay takes effect. From your Tello Dashboard, select Change Plan, build the plan you want (even if it's the same plan you had), and select. Scroll down to payment terms, and select "Pay With Tello Dollars". Confirm.