Just look at the box, then grab the same one on your next grocery run. Do the same with her other feminine products. Put them in the cabinets where they go. Act like it was nothing. Make her wonder how long it was since she bought them herself.
Advanced mode: Figure out her clothing sizes and what colors work for her, what fabrics feel nice, then get her a cute blouse or dress. Don't make it a big deal.
See what happens. One of my best friends mastered this. Dude cleaned up in college.