How to write a thank-you card for a gift you haven’t received yet?

I would wait to send the card out to her since you would want to thank for the money and gift. She will receive her card a bit later though you did receive the parcel. She may have purchased something personal to give you two apart from the registries possibly.

You always wait until you get the gift from her? You just got married a few weeks ago, there's still plenty of time to send out the thank you cards! My husband and I are sending ours out probably next week or the week after and we got married the beginning of August

She could have looked at your registry and found that item somewhere else? I would just say thank you so much for the lovely candle, the money and looking forward to the 3rd gift, that once it arrives, you'll call her personally to thank her. That her generosity means a lot to the two of you. Just a simple thank you goes a long way to your guest.