eXpansys offers a $50 SIM 4G Refill Card T-Mobile for $35 plus $3.99 for shipping. That's 30% off and tied with our mention from last week as the best deal we've seen on a $50 refill SIM card for T-Mobile alone. (For further comparison, that's the lowest total price we could find for a comparable regular refill, without the SIM card for T-mobile by $17.) It includes a T-Mobile SIM card and unlimited text, talk and web. It requires a new connection (meaning you'll need to get a new phone number) to receive the discount. The refill may only be used with the included SIM card.
Of note, this SIM will work on any T-Mobile device that uses a SIM card.