They booked a visit because the downstairs bath is slow. You have not opened anything yet. That is the advantage. Your assistant drafted a friendly confirmation that says you will "take a look." A look without a scope is how a two-hour call becomes an argument about what they thought was included.
This is not the mid-renovation ambush, where the wall is already open and the crew is waiting. This is the planned job. Write the fence while you still can.
In, out, and what you will ask before you go further
Name the decision. "I need you to confirm we open the access panel and replace the trap if that is the restriction, and that we stop and write you before we open the wall." Say what the visit includes, what it does not, and the likely fork if the first look matches what you expect.
Give options they can choose now: repair the fixture only, add a camera if you cannot see, or hold the visit until they can be home. Then your recommendation. Do not attach the quote sheet. RelayLink does not carry attachments. Describe the work in the briefing — which bath, which fixture, which access — so they can answer from a desk.
A homeowner who says "just fix it" has not approved a wall. Put that sentence back to them as a choice.
Write the assumptions you will otherwise discover on site
You assumed they still want the same fixture, not a swap. You assumed the shutoff works. You assumed someone will be there to show you the second bath they mentioned. Put each on its own line. A correction before you load the van is cheap.
If you find something once you are there, that later message is the other article — options, cost in time, what it rules out, a deadline they can meet from work. Do not pretend today's planned scope covers that surprise. The value of writing now is that both of you have a baseline when the surprise arrives.
They do not need an account; you still approve the send
Most homeowners are not on RelayLink. The briefing travels as email and a link. They can reply from the mail. If this address already has an activated account, you need an accepted contact pair for a new thread. You review the wording before it leaves. A nickname you use for the house stays on your side.
If two people own the house, one briefing still goes to one mailbox. Say the other person should see what you will open.
After they confirm
Write back the in and the out in the same words, and ask them to correct it. Then do that job. If you need to go further, stop and write the fork. The planned visit stays honest only if the surprise gets its own ask.
The van can still find something. The email should not be where they first learn you were always going to open the wall.