[myobripicusni]

I'm looking into this too, and found the Exchange Online Plan 1 which is a dollar cheaper at 4/user/month.

I'm just trying to figure out if you can add multiple domains and aliases (that can be used for sending) with a single user license.

[rootsudo]

you can if you are on a business o365 plan, so if you get Exchange Online Plan 1, you should be able to do so.