Same here. Topped up now multiple times with the prepaid (and other) cards, the first time is a month ago by now and I wasn’t charged any fee. And that still holds true after having received the monthly bill. Therefore, I am inclined to believe too that this is the best way to top up at the moment, at least until UBS changes something
However, it might be the case that by now the payment processor is situated in Switzerland, as I remember someone else saying that his/her top-up with the Migros Card was without any fee for spending CHF abroad too, despite the fact that it “normally” should.