The type of card does not correlate with the number of stamps you put on the envelope, the number of stamps is determined by the mailing distance. If the card is abnormally heavy, you might want to send it from a post office instead of dropping into a box just so you can verify it will reach it's destination with the amount of postage you've provided.
The weight should be around $3...so around six letter stamps.
Jennifer take it to the post office please
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