I have found printable invitations and thank-you notes both convenient and inexpensive. Once you have the supplies you need on hand it is a snap to print out what you need and and get your notes sent out quickly.
Here is a list of thank-you note resources you might want to consider purchasing:
White Card Stock (110 lb. recommended)
Avery 8160 Address Labels (both for printing addresses and for printing out the included cute labels/stickers in the packets). These are for inkjet printers – if you need labels for a laser printer, you can purchase 5160 labels.
Avery 22807 Round Labels (for printing the fun labels/stickers included in the packets)
A top-notch scissors (I use the Cutter Bee Precision-Cut Scissors but you can use any that work well for you)