I sent mine last week, and it hasn't received by AADSAS yet. So I'm not 100% sure this is the right way.
1. download the pdf file of letter matching form from AADSAS website.
2. upload it on My Documents at interfolio.com
3. make a delivery with both matching form and letter of rec to AADSAS.
I talked to a customer service representative at interfolio.com, and she told me to do it this way.