Four months ago they may have been looking for work, now they're busier. Or, who knows?
They can price their work any way they want. Especially if you didn't accept the "estimate" (whatever specifically that means) at the time.
No, GGI is busy all the time, they always have a long wait - months rather than weeks. And, I've heard they do good work, though perhaps not as customer involved as TheSigArmorer. GGI has also taken and early interest in support of the P320, so has experience and stuff developed for that gun, probably more so than other aftermarket vendors.
But, unless they have a little line on their order form that says, "Our quotes are estimates only, we charge whatever we wish when we are done and you must pay that if you ever want to see your gun again!" (or words to that effect), they need to honor their quote. It's good for customer relations and it's the right thing to do.