There are many stores out there (e.g. Coffee shops) that have a hechsher, but are Chalav Stam. To get around this issue, some of these stores offer the option to request Chalav Yisroel milk for those who need it.
For visualization purposes: the store carries a Chalav Yisroel milk bottle. When a customer requests it, the storekeeper will substitute the standard milk with the milk from the Chalav Yisroel bottle.
My question, though, is whether this actually helps the Chalav Stam/Akum issue? After all, if the milk bottle is sitting in the store throughout the day, do we not suspect that the storekeeper (who's presumably a gentile) may mix the milk? Shouldn't this "Chalav Yisroel" milk once again become Chalav Stam/Akum milk?
Clarification: My question is due to my understanding that milk in particular requires an extra level of supervision. For example, in order to make milk Chalav Yisroel, it is required to have a Jew supervise the actual milking process. This is very much unlike other products where concepts such as נכנס ויוצא (perhaps best translated as "spot checking") do work.
So: the hechsher I referred to in this case is merely serving the purpose of providing a basic level of Kosher supervision - and confirming that the milk in question does indeed qualify for the "Chalav Stam" leniency. The milk in question, on the other hand was at one point Chalav Yisroel (according to the label), but is now sitting in the store - open and unsupervised. Does this change its status?
[If it helps: it's also possible to construct this case without any hechsher at all: simply pick your favorite unflavored coffee scenario that's permissible anywhere and add a Chalav Yisroel milk bottle. Was just trying to boil the case down a bit...]