You cannot do Saturday 9–12 at the front desk. You posted in the volunteer chat. Eight people saw it. Two said "sorry." Nobody took it. On Saturday the coordinator will discover the gap when the door opens.
A shift swap is a yes from a named person. A list post is a hope. Hopes do not unlock the building.
Ask one person
"Can you take Saturday 9–12 at the front desk, same building, I will leave the key with Jan, yes or no by Thursday 6pm. If you cannot, I will ask Priya next." That is a decision.
Give the facts they need to say yes: day, clock, role, who else is in the building, who they call if something breaks, whether they have done this shift before. Cut the story of why you cannot come. One clause is enough — a trip, a conflict, you are out.
Use their name. Not a private nickname. If they have a name they use on the roster, that is the one.
Do not ask them to take Saturday and cover your Thursday phone shift. That is two swaps. You will get the easier one and a still-empty desk.
They will answer from email
They need no account. Most volunteers are not connecting an assistant. The mail is the product. They reply from the mail or the page. You approve the wording. Your assistant does not staff the desk as you.
If you use Claude and the coordinator uses ChatGPT, the volunteer still reads one briefing.
No handbook file. Write the one shift. If a one-page role sheet already lives where volunteers look, name that place.
If they already have a RelayLink account, you invite first. The invitation carries no note. The shift ask comes after they accept.
What a sufficient reply is
"Yes." "No, ask Priya." "Yes if I can start at 9:15." One line. Say that is enough.
Then a default you can still act on. "If I do not hear by Thursday 6pm, I will ask the next person and I will not chase this." That leaves you Friday. A default that assumes they will show is how a door stays locked.
Follow up once only if the clock moved. A bump is a second interruption.
When they say no, write the next person the same day. Do not return to the list in between. The list is what failed the first time.
What this is not
It is not a resignation. If you are leaving the program, that is a different letter to the coordinator, with a last date.
It is not a place to process burnout in the same paragraph as the clock time. The person you are asking needs a shift they can see. The coordinator can get the longer letter separately.
If your assistant already has the roster, let it draft the one-person ask. You read the name and the clock. Then they can answer without a group chat.