Hoo Boy, getting the shipping charges to work right on a web store ordering system is a challenge, but I think I've got it pretty close.  First, please notice that you can go through the ordering process far enough to get the shipping charges without actually placing an order, so you may want to go through it a couple times trying different shipping services to see which works best for your situation.  Our shipping charges include:

Actual freight charge from the carrier.  For UPS, this is gotten from a UPS server as you check out, based on weight, the service you choose, and your location.  It is accurate.  For Priority Mail, the charges are based on tables that I put together, based on Postal Rate Tables.  The postal service does not have an online server like UPS does, and postal package charges are quite complicated, so my tables have to make some assumptions.  They may not match the Postal estimator at WWW.USPS.COM exactly, but they are pretty close. 

We have a handling charge to cover the cost of materials (we spend hundreds on boxes and filler material, on most orders the box and filler costs nearly $1.50), and about 50 cents to pay the packer.  We reduce these charges by $1 for small Priority Mail shipments, as we get the boxes for free from the Post Office. These charges are per order, not per item, as it doesn't take much more time and materials to pack two or more items than it does to pack one.

Hints:

We have a selection for "Customer Pickup".  Don't choose this unless you plan on meeting us at the shop to pick your item up.  Check with us first, as we don't keep regular hours at the shop.  This really only makes sense if you are picking up something big and live in the area, or want to see the item first. 

Priority mail is a good deal on light items, then it's fast and cheap.  If you are far enough away that they have to put it on a plane to get it to you in a couple days, and it weights more than a few pounds, then Priority Mail gets expensive fast, as it is an air freight service, not ground.  It is still the best, if not the only way to go, for Alaska, Hawaii, APO, FPO and US Territories however.  Please note that the Post Office has a size limit.  We cannot ship 19" monitors postal.  Two things you should know:

UPS Ground is reliable, but the time it takes depends on the distance, as it travels by truck, train, whatever.  You will find that if you live close to Chicago, there is no point to paying for UPS two day or three day service, as you will get it that fast anyway.  You can verify delivery time by checking the ship time at WWW.UPS.COM from a shipping zip code of 60544 to your address. Things you should know about UPS: 

Lower 48 States

You've got all kinds of options, feel free to try them all before placing the final order, to see which way is fastest and cheapest.

Alaska, Hawaii, APO and FPO

I think you will find that Priority Mail makes the most sense.  The system should not allow you to order UPS Ground, UPS doesn't offer it.  

United States Territories

You will probably find your best deal is Priority Mail.