"Relax. I Love You." ...
"Relax. I love you." ... nice, but as a man I feel it is somehow missing the mark. Also, you can market the "love" angle throughout the year: Valentine's Day, birthdays, anniversaries, honeymoons, pre-weddings, post-natal, pre-natal, Mother's Day, etc. Since you are doing a special order, consider how you might make the "love" theme more universal so you can continue using the remainder through the rest of the year.
Here are a few alternative wordings that popped to mind:
Because you deserve the very best...
Relax, my Love...
A Valentine Massage...
A Massage for the One I Love...
A Massage from the Heart...
If you find a wording that you'd be happy to continue using year-round, consider having the gift cards printed on double-sided business card stock. Customize both sides and get $250 for a reasonable price, more if you buy extra right away. You can always stick them into greeting cards with a dab of rubber cement for a more elaborate presentation, yet the recipient will only have to carry the (business) gift card.
Hope that helps!