The host can decide whether he or she would like to make the Service Plus is optional or mandatory. They will be able to set the default but they can also change it for a particular exchange.
Setting Service Plus as optional for a particular exchange:
If you would like to set the Service Plus as optional for a particular exchange,
1) Go to your messages by clicking on "Messages" at the right side of the top bar.
2) Click on the relevant conversation for which you would like to set the Service Plus as optional.
3) Once you are in the conversation, beside "Service Plus", select "Optional".
Setting Service Plus as optional for all your future exchanges as a host:
If you would like to set Service Plus as optional for all your future exchanges,
1) Hover your mouse over "Profile" at the top right hand corner of the screen and click on "My house" from the dropdown list.
2) Once there, click on "Optional" under Service Plus, as show below:
Here, Service Plus is being set as mandatory:
And here, Service Plus is being set as optional:
Don't forget to click on "Save changes" once you are done!