The escrow should be released when people have received their product, so I don't see how you would do this.
Are you planning on funding this all by yourself?
As I have understood, you was against pre-order due to big risks for customers. This problem can be solved with ESCROW - you order calendar, send money to escrow, when I send you calendar, money from escrow goes to my wallet .
Simultaneously as I see amount of preorders I know how many calendars I should produce and Im sure I will receive this money.
Pre-order minimizes my risks as producer too, lets say I start pre-order in October, receive for ex 100 orders, I will have 1 month for production and can ship calendars from November. So in case I will be able to cover this production volume myself, I can borrow money, knowing funds on escrow.
This approach allows to minimize initial investments, what I should definitely do on my own - produce 1 box.
Generally speaking, if this idea will be beloved by community members, I think here are some people who can contribute in design and production and other fields.