Why not contact the rescue agency and discuss your desire with them. Your idea sounds like the recipe for mishaps to occur, even with the best of intentions in mind.
I can tell you from experience that when it comes to selling items from one location and the money having to go somewhere else, when multiple people are involved in the process, it's can be easy to lose product, lose money, and alienate your customers.
How will you know when a customer has paid for the product in order for you to ship it if all the money is going to the rescue paypal account? They surely aren't going to give you access to it, and it might be a time waster for volunteers to have to review the paypal account everyday and notify you if one bar of soap sold.
Maybe I'm missing something, and I don't want to sound like I'm poo-pooing your idea, because I'm not. It's a very thoughtful thing you want to do. Why couldn't you give the soap directly to the rescue and let them take care of payments and shipping the soap? You can continue to be the PR person to promote the soap and the organization on the forums you belong to.
It might make even more sense and look more legitimate to readers if you "brag" to members that the rescue organization is now selling your soap, and hopefully it will drive more people to their site, which could result in more adoptions (or sales of soap), and possibly more income if they earn money from advertisements on their site.
Also, you would need insurance unless the rescue organization is willing to accept liability for your donation and they put that in writing.