You can order by PayPal using
https://www.regnow.com/checkout/cart/add/37048-1?quantity=XXX
where XXX is sum you want to pay / 2
for instance to pay $100, please use payment url:
https://www.regnow.com/checkout/cart/add/37048-1?quantity=50
to pay $150, please use payment url:
https://www.regnow.com/checkout/cart/add/37048-1?quantity=75