You're not really asking for a "thank you" you're asking for acknowledgement that a package was received. Should be a given in an organization working from donations.
I used to send quilts to an organization that gave quilts to children. Until they put out a picture asking for volunteers to get the quilts to the kids and showed a picture of a storeroom with HUNDREDS of quilts. Wow. I work two jobs and felt so good making those quilts. Did not expect them to sit in a storeroom. (I don't live close enough to have helped with distribution.) Figured that charity had done will with letting people know the need and had many people sewing for them. I'm now making small quilts for a local family who are just getting started in foster care. My goal is to keep them supplied with a couple of quilts so that each child gets one to keep - and gets to choose which one they want - even if it's just "this one" or "that one"! We want to give - we each just need to find the right fit for the giving.