This sentence does not appear to be correct as is. Note that verb-ing modifiers when not preceded by a comma, modify the preceding noun. In this case since "thanking my 8th grade teacher..." is not preceded by a comma, it modifies "card". And for the modification to be correct, the verb-ing modifier should state a property or characteristic of the preceding noun. For example:

I sent a card containing letters inscribed in gold. In this sentence "containing letters..." describes the card appropriately.

However, in the sentence in question, "thanking my 8th grade teacher" is not a characteristic of the card. It is basically an emotion expressed by the person who has sent the card.

The correct construction should have a comma prior to the verb-ing modifier.

Another correct construction can be made by replacing "thanking" with "to thank"
I sent a card to thank my 8th grade teacher...

If you would like to review the verb-ing modifiers, you can access the concept titled "Modifiers-Verb-ing" in the Free Preview of e-gmat SC course.

Thanks,

Payal

Yes that is correct.