It appears the t-mobile has suggested that your basis is their cost to quire your shares, though they do not say this is 100% certain. I think this is because they are treating the cost of aquiring the shares as a discount on your mobile service. Therefore your mobile service cost less and the amount less was spent (for you by T-Mobile) on the share(s) of stock.
Given that and the small amount I would suggest not adjusting. That is not 100% certain to be right.
[This assumes these are not shares for referrals. Those would seem to create income when received and have a basis of that income recognized.]
We anticipate that your adjusted tax basis in your Shares will most likely equal the purchase price (calculated using the method described under the heading “Price of Shares” in this prospectus supplement) paid to acquire those Shares in the open market for delivery to you.
https://www.reddit.com/r/tmobile/comments/5mju78/tmus_loyal3_stock_rewards_taxes/
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