Operation Birthday Surprise went off pretty well. The exchange went something like this...
I offhandedly walked into the bedroom with the stack of boxes that were supposedly "guitar parts" and I said, "These are actually for you."
The reaction was something like this:
"But you already got me something!" (Which I did, a new iPhone)
I told her I wanted her to get something on her actual birthday, but I know she hates surprises (which she said "I DO hate surprises!") and I said, "I think you love surprises, but you just hate not knowing what it is... So go ahead..."
I got the right t-shirt. Apparently she had tried to buy it again but could not find it... I spent about 3 hours paging through every damn t-shirt on amazon.com to find it.
I think she enjoyed her birthday. Mission accomplished.