Using Excel to determine a shipping schedule

I am attempting to use Excel to design a schedule for shipping and am having some real difficulty. I've spent the last two days trying to hammer out a solution but I'm not finding one.

Here are the details:

• I have a list from the customer of when they want the product shipped to them. They will receive (I will send) one shipment every month, and each month there is a required number of units.
• I can ship units early but I can't go over or under the final totals at the end of the year for the number of units.
• The minimum number of units that can be in a single shipment is 15. I can choose to ship zero units in a month if the customer already received the ones they needed early.
• I have to minimize the number of units shipped at any given time, but only if the customer has what they need already.

Here is an example of when the customer wants to receive units and how many:

Jan 10
Feb 20
Mar 15
Apr 30
May 50
June 75
July 100
August 110
September 100
October 70
November 40
December 20

What I want my output to look like is to tell me if I ship units in a given month and how many to ship, while following the above rules.

EDIT:

The system must also be able to handle this scenario:

Minimum shipment of 25 units.

Jan 10
Feb 0
Mar 12
Apr 0
May 5
June 0
July 0
Aug 5
Sep 5
Oct 13
Nov 5
Dec 5

Is there a max of units the customer want to have?
Is there a max of units you can ship at once?

Otherwise you do everything in 1 shipment that is cheap and easy

Take a look at this.

Please see the attachment. I've used mixed relative and absolute addressing to give running totals. If you enter your numbers in the shipping column, you will be able to see if the balance is positive or negative.

Is this a problem with my imposed minimums and I'll have to accept that inaccuracy?

That is correct.
You will compensate it the next year.

If minimum shipment is 25 and december needs 20 then you shipment in november must be 5 less
But then you don't heave enough in november.

So it is impossible

A little adjustment.
Don't deliver in december and deliver 20 more in november
Take a look at the formulas from nov and dec.