Stumped. need to add option to checkout or shipping

Hello,
I have not had success finding a solution to customizing my checkout and/or shipping section.
My site is https://www.igofresh.com. Linux hosting and ZC 1.3.9g.
I deliver groceries in 3 time blocks; Morning, Afternoon, and Evening.
I am running the Zipship add on as my areas for delivery are specific to zip code.
Somewhere during checkout I would like to offer the customer the ability to select, via drop down or radio button, which delivery block they would like. It seems simple but I have not found any similar case studies to draw from. Any help is appreciated.

To clarify, I don't need to involve Zipship, I can work with table shipping too. I think I would need to specifically work in 'checkout_shipping.php. I have had no success modifying the comment table. I don't know php well enough to write this on my own but replacing the comments field with a drop down or check box that drops the field name into comments would work. Anyone know how that would work?

Ok to this point I'm still in this alone, but I'm sure someone else may need some of this info so I am posting what solutions I find.
So far I found how to turn the checkout comments into a drop down box via this thread;
http://www.zen-cart.com/forum/showthread.php?t=107227

Now if I can make it so that drop down (where the customer can choose one of the three delivey time slots) is 'required' before moving on to step 2 of the checkout. I don't want them to miss selecting a delivery time.