DD has a beaker with a flexible (silicone?) straw that came from Tesco (own brand I think). It is free-flowing and has a flip top. I slightly warm the milk in the microwave without the top on and it is fine -never overflowed.
Both of mine went straight from boob to open cup with meals etc, but for whenever this is inconvienient they have had anyway up cups.
I'm not sure the heating thing is so much for the top popping off as the chemical content of the plastic can release something (phps? or summat) when heated and this is thought to be carcanogenic (or summat) sorry I am not too hot on the technical stuff.
Some cups are safe, lots arn't you need to research, there are lots of threads on here on it.