No they will not fill growlers that are not purchased from there. I think the smallest one is $7.99 and the larger ones are $10.99. I know Mully's will fill other growlers but they have to be growlers purchased in Maryland. Apparently that is a Maryland law/requirement that breweries can only fill growlers purchased in Maryland. We have some growlers we bought from Founders and have been unable to fill them in Maryland. The DC breweries were not a problem though.
As far as I know, Maryland as a state doesn't govern that, it's left to the individual counties liquor boards. Below is Calverts permit request, the only place it really defines whats an acceptable "refillable container", note item B, which after re-reading, only seems to say that it has to be marked by who sold the container, not who's filling it. So, as long as it's marked by whomever sold it (and theres no requirement that it be from MD that I can see) and has both the health warning, cleaning instructions, and the other two things, they cannot refuse to fill it.
Looks like the wifes Cricut has it's first beer related project, to add those stupid cleaning things.
To be used as a refillable container, under the previous paragraph, a container shall;
a.) Be sealable;
b.) Be branded with the identifying mark of the Permit holder who sells/sold the
container.
c.) Bear the federal health warning statement required for containers of alcoholic
beverages under 27 C.F.R. 16.21
d.) Display instructions for cleaning the container
e.) Bear a label stating that;
i) Cleaning the container is the responsibility of the consumer
ii) Contents of the container are perishable and should be refrigerated
immediately and consumed within 48 hours after purchase