Deposit

You might have to fight Booking.com over this - if so, fight them, do not take no for an answer

We have a similar setup with our properties - guests arrive and let themselves in - they may be delayed on their journeys and it is unfair to keep them waiting for us, and we can't have someone waiting at each property for whatever time they arrive. Also, cash is not "safe" - damage deposits should always be taken and refunded electronically so that there's a proper record - this protects guests and owners.

We have an automatic message sent upon booking confirmation - it asks guests to pay the damage deposit by bank transfer - it includes the bank account details (we have a separate bank account for BDC damage deposits so we can find the payments easily) and we ask guests to give their bank account details so we can refund them easily.

Then ask for a change to The Fine Print in Property > View Your Descriptions. This information is visible on the Booking Confirmation page that guests often print before they travel. The change should say the damage deposit must be paid before directions / key safe access are given and that guests should check their inbox for messages.

Booking.com will refuse to allow this text again and again. Keep your property closed and tell them you cannot open it for bookings unless you can protect your guests. It's not just the current guests - you need to ensure the property is undamaged by one guest, therefore you are also protecting the next guest.

Fight them until they give in. I normally have to make the change requests several times. It's when I say "please pass this to someone with brains" or "if you are just a minion, do not reply, please pass this to a manager" or something.

If they don't give in, use AirBnB. Seriously, don't risk your property, don't take risks with future guests. Keep fighting Booking.com - eventually they will give in.