Down payment / Deposit / Reservation fee question

So I am trying to figure out how to set up an initial down payment option for customers. 

I am building a small batch of boxes, I need the customers to put a deposit down and pay the remainder plus shipping once the boxes are complete. 

Example A - Customer wants a box made out of maple, the deposit to get them on the list is $100. The total cost of the box minus shipping is $375 so once I am done building this batch of boxes I need them to be able to pay the remainder $275 plus whatever the shipping is going to cost to their location. 

Example B - Customer wants a box made out of walnut, the deposit to get them on the list is the same $100. The total cost of the box minus shipping is $425 so once I am done building this batch of boxes I need them to be able to pay the remainder $325 plus whatever the shipping cost is to their location. 

So the way I have it working in my head is, I put up the item, limit the quantity to the amount I want to make, once they put down the deposit fee and all of items are reserved I have a list of all the people who put down the deposit and their email, I build the boxes and send out a mass email letting them all know it is completed and they need to finish the transaction by paying the fee + Shipping and I get them shipped out with a return email with the tracking numbers...???

Any help or keywords that I can search or research would be greatly appreciated!

Thanks

Erik

I have the same issue as the OP with the complication of having variants for each product which determine the final price. This makes the workaround above unusable.

 All I seem able to do is require payment in full or ask customers to email if they would rather make a deposit. This is far from ideal. 

Has a workaround I can use or a proper feature been added yet to address this? Maybe it's technically hard, but I'm surprised it is lacking. 

No reply. So I'm infer this is a feature that won't be added. It is important for my business, so next year I'll move on from Squarespace. What a shame.