I am now making assumptions.
I am assuming that:
the manufacturer electronically transmits to you the tax invoice at the start of the first day that shipment can commence;
you pay the invoice with guaranteed funds that same day;
shipment is initiated the day after payment of the tax invoice is received;
delivery occurs two days after shipment is initiated;
you require one day for inspection, installation, and testing after delivery; and
you can use the equipment the day after testing is complete.
Days 1 thru 30: -15%
Days 31 thru 34: - 75% (15% + 60%)
Days 35 thru 60: + 25%
30(-15) + 4(-75) + 24(25) =
- 45 - 300 + 600 = 255
255/60 is about 42%
So I guess you could say that this is equivalent to credit of 42% for 60 days compared to 100% to commence
But I think it is better to think that you are clearly financing them for 30 days at 15% and they are clearly financing you for 24 days at 25%. The whole process of paying an additional 60% before delivery seems one-sided to me.
If they are concerned about getting mostly paid before physical delivery, I’d think about some arrangement (eg documentary letter of credit) whereby the manufacturer bills you for the 60% when shipment is initiated and you pay it with guaranteed good funds to get the shipment released for delivery. As it is, you are paying 75% just on their word that there will be a timely delivery.