I'm one of those people who would feel like I needed to cover the bases, on the off chance she really has just had a terrible time arranging the pick up, or is reliant on others who are unreliable to help her get it.
She may just not know how to say, "I've reconsidered and don't want it." But, it could be crazy times for her... just never know. I would simply tell her that you wanted to give her a head's up that you can't store it any longer and will be delivering it to (whatever charity you decide on,... I think the foster facility sounds like a great option, personally) on Saturday. If she still wants it, she'll need to pick it up before then, and no big deal if she can't... you have it all arranged on Saturday to go to X (the charity). Then don't worry about it again. You probably won't hear any more from her, and she may even say what a great idea/gesture that is! ;)
Regardless, it lets you both off the hook and keeps things amicable between you.