However, I discovered that it is possible to buy a collapsible travel mug made of silicone that folds up small enough to fit in my purse, so I could carry it with me at all times. So I vowed at the time of my post that, if no one else got me one for my birthday, I'd get one for myself. And now, a month later, I not only have one but have finally had an opportunity to use it, so I can report on the success of the experiment.
When I deployed this for the first time at Starbucks yesterday, I was pleased to see that the staff didn't bat an eye at my request to use my own mug. It even made collecting my order a bit easier for me, since I could see at a glance which drink was mine. It held my tall peppermint mocha with plenty of room for the whipped cream.
Drinking from the mug was a little bit more complicated than drinking from a disposable cup. Since the mug is made mostly of soft silicone, the only part that's rigid is the plastic sleeve that fits around the middle. So you have to take care to grab the cup only by that rigid middle section, not by the soft part, or you'll end up squeezing it too hard and splashing coffee everywhere (or possibly burning yourself, since the silicone parts also don't provide as much insulation). But I found this quite easy to adjust to.
I found it easy to drink from the cup with the lid off, but I couldn't put the sippy-cup lid to a good test, since I'd just come from the dentist and couldn't pucker up my mouth properly. I tried it later on with some water and it worked fine, but I still don't know how good it is for protecting your mouth from scalding hot liquid. The sippy top also has a little silicone tab that you can close up to prevent splashing—something you can't do with a disposable Starbucks cup.
Cleaning the cup was also quite easy. I just swiped it with the dish wand, then rinsed it, same as any other mug. However, I must admit that the silicone material tends to hold on to odors a little bit. When I got the cup out just now to test its capacity, I found that if I stuck my nose right inside, I could still faintly detect the odors of chocolate and peppermint. Not that I mind this, but it might be a bit incongruous if I wanted to fill it up with, say, hot cider on my next visit. But since I expect to use it pretty much exclusively for coffee, I think it will be fine. (I also didn't notice that the water I drank from it had any discernible hint of peppermint mocha flavor.)
So based on this first use, at least, I have no real complaints about this collapsible mug, and I consider it the ideal solution to my disposable-cup problem. Of course, only time will tell how durable it is, but if this paper from Tufts is to be believed, as long as I get at least 17 uses out of it, it should have a lower carbon footprint than the paper cups and plastic lids it replaces.