I agree with the simple . . . maybe just a football and a cute sports-type font. I make my invitations 4x6, so that printing is inexpensive and quick (usually upload to my local pharmacy that has 1 hour printing and zip by and pick them up). Can use regular legal size envelopes if in a pinch for time or money, but Target has an envelope size and the 4x6 print will just slip right in.
If she designs it 6x4, and could find a goal image from stock exchange or something, that might be cool to blend in the background.
Don't really have any catchy title/invite phrases right now . . .