You could also compress it with 7zip and hope it turns out below 2 GB. However I doubt 7zip is going to be that much more effective. Typically you can only expect about 10% better than regular zip but here's to hoping?
Other than that, 2.5 GB is quite a lot for an indie game and I am reluctant to believe that 500 MB can't be freed somehow until support replies.
Butler compresses somewhat better than standard LZMA so maybe you can get away with just saving 400 MB. Which engine are you using?
https://docs.unrealengine.com/en-US/TestingAndOptimization/PerformanceAndProfili...
Did you enable shared material shaders shared material library code?
Did you remove the prerequisites etc?
Did you remove unused plugins?
There's lots of things you can do to reduce the game size, depending on your game of course there is a limit. All we know so far is that it is 2.5GB and Unreal Engine.
Doing these three will likely remove about 100-200 MB, still above target but closer. Make absolutely sure that you are only packaging maps that you need and I expect that would take care of the rest.
