The default configuration of Odoo.sh instances should work.

The catchall address must use a domain which is associated to Odoo.sh database.

e.g.
If your database can be accessed using yourcompany.odoo.com,
the catchall domain can be @yourcompany.odoo.com.

This is the default configuration done for new Odoo.sh databases.

If you setup a custom domain in your Odoo.sh project settings, you can use that domain as well.

e.g. 
you configure mail.yourcompany.com in your custom domain in your Odoo.sh project settings,
then you can use @mail.yourcompany.com.

Of course, in your domain registrars, you need to create a MX record mail.yourcompany.com pointing on yourcompany.odoo.com.

That said, after viewing your database logs, I do not believe the issue comes from a mis-configuration of your email alias. Indeed, evidences the replies arrived can be found in your logs, but Odoo couldn't find the original message to which this reply is addressed:

/home/odoo/logs/odoo.log.1:2018-07-13 15:47:38,036 4 ERROR xxxxxxxxxxx-xxxxxx-odoo-com-xxxxxx-xxxxx odoo.addons.mail.models.mail_thread: No possible route found for incoming message from "Bruce Letterle- xxx xxx Xxxxx" <bruce@xxxxxxxxxxx.com> to "Xxxxxx Xxxxxxxxxx" <catchall@xxxxxxxxxxxxxxxx.odoo.com> (Message-Id <1531496852.795812487@mail.emailsrvr.com>:). Create an appropriate mail.alias or force the destination model.
/home/odoo/logs/odoo.log.1:Traceback (most recent call last):
/home/odoo/logs/odoo.log.1:  File "<string>", line 6, in <module>
/home/odoo/logs/odoo.log.1:  File "/home/odoo/src/odoo/addons/mail/models/mail_thread.py", line 1300, in message_process
/home/odoo/logs/odoo.log.1:    routes = self.message_route(msg_txt, msg, model, thread_id, custom_values)
/home/odoo/logs/odoo.log.1:  File "/home/odoo/src/odoo/addons/mass_mailing/models/mail_thread.py", line 31, in message_route
/home/odoo/logs/odoo.log.1:    return super(MailThread, self).message_route(message, message_dict, model, thread_id, custom_values)
/home/odoo/logs/odoo.log.1:  File "/home/odoo/src/odoo/addons/mail/models/mail_thread.py", line 1184, in message_route
/home/odoo/logs/odoo.log.1:    (email_from, email_to, message_id)
/home/odoo/logs/odoo.log.1:ValueError: No possible route found for incoming message from "Bruce Letterle- xxx xxx xxxxx" <bruce@xxxxxxxxxxx.com> to "Xxxxxx Xxxxxxxxxx" <catchall@xxxxxxxxxxxxxxxx.odoo.com> (Message-Id <1531496852.795812487@mail.emailsrvr.com>:). Create an appropriate mail.alias or force the destination model.

What's important when a customer reply is sent to your database is:
 - the fact it uses your catchall address as the recipient (catchall@yourcompany.odoo.com),
 - the message as in its `references`header the `message-id` to which it replies, so Odoo can find back the original message to which this email is addressed.

The fact Odoo couldn't find a possible route for your message can have multiple causes:
 - The document from which the original mail was sent has been deleted,
 - The customer used the button "New mail" and copy/pasted the recipient instead of using the "Reply" button, in such a case the `message-id`is not set in the `references` header of the email,
 - The email client does not respect the email protocol and omit to set the `references` header correctly,
 - The email server from which was sent the reply drops the `references` header.

From experience, the most-likely cause is one of the two firsts stated above.