Valentine's Day in the early 19th century

From Cupid's Annual Charter

Loretta reports:

Mid February has been associated with some sort of mating ritual, going back, apparently, to ancient times.  By the early 19th century, Valentine’s Day was well established as a day for romance or at least romantic hopes.

According to Hone’s Every-day Book, (1826), “Two hundred thousand letters beyond the usual daily average, annually pass through the twopenny post-office in London on St. Valentine's Day.”

The February 14 entry continues, quoting Charles Lamb:   " In other words, this is the day on which those charming little missives, ycleped Valentines, cross and intercross each other at every street and turning. The weary and all for-spent twopenny postman sinks beneath a load of delicate embarrassments, not his own. It is scarcely credible to what an extent this ephemeral courtship is carried on in this loving town, to the great detriment of knockers and bell-wires.”

You'll find a good deal more about Valentine's Day in Hone, including a large dose of poetry.

Cupid's Annual Charter; or, St. Valentine's Festival (1815), contains poetry for every possible state of mind and recipient, including valentines to misers and prudes.

Here’s a sample from its many offerings:
Valentine.
You are not handsome it is true,

From The Sentiment of Flowers

Yet you’ve many charms for me,
Believe me that my vows to you,
Is from deceitful falsehood free.
Tis not the feature of a face
I seek for in my future wife
Ah no! for other charms I seek
To sweeten my domestic life.
Say then my fair will you be mine,
And wed your faithful Valentine.

Answer.
Other charms I have 'tis plain,
You are a fortune hunting swain;
And for better, and for worse,
You'd have me for to hold my purse;
But this fine scheme it will not take,
You shall not cause my heart to ache;
My person's plain full well I know,
But you'd no need to tell me so;
So my ill manner'd Valentine.
Your sordid offer I decline.

The poetry might inspire you to create your own card.

The less verbal might decide to say it with flowers.  In that case, I recommend you first consult The Sentiment of Flowers: or, Language of Flora, (1840).  Clematis or madder, for instance, would not be the best choices.