Is there a way I can just charge all my customers a single price? This way I'm never overpaying for shipping if I have to ship an item coast to coast.

The USPS offers a wide variety of Flat Rate boxes and Envelopes. Long story short, if your item fits in the box and is less than 70 pounds, then it'll ship to any zone for one flat rate. These packages fall under the shipping methods of Priority Mail Express and Priority Mail. Here's a list all all the current sizes the Post Office offers: (Just know with Panda you'll receive a discount on all these packages)

Priority Mail Express:

• Flat Rate Envelope
  12-1/2" x 9-1/2"
• Legal Flat Rate Envelope
  15" x 9-1/2"
• Padded Flat Rate Envelope
  12-1/2" x 9-1/2"

Priority Mail:

• Flat Rate Envelope
  12-1/2" x 9-1/2" or smaller
• Legal Flat Rate Envelope
  15" x 9-1/2"
• Padded Flat Rate Envelope
  12-1/2" x 9-1/2"
• Small Flat Rate Box
  8-5/8" x 5-3/8" x 1-5/8" Inside
  8-11/16" x 5-7/16" x 1-3/4" Outside
• Medium Flat Rate Boxes
  11" x 8-1/2" x 5-1/2" Inside
  11-1/4" x 8-3/4" x 6" Outside
  13-5/8" x 11-7/8" x 3-3/8" Inside
  14" x 12" x 3-1/2" Outside
• Large Flat Rate Boxes
  12" x 12" x 5-1/2" Inside
  12-1/4" x 12-1/4" x 6" Outside
  23-11/16" x 11-3/4" x 3" Inside
  24-1/16" x 11-7/8" x 3-1/8" Outside
• APO/FPO/DPO Flat Rate Box (USA Military Bases)
  12" x 12" x 5-1/2" Inside
  12-1/4" x 12-1/4" x 6" Outside

If your item fits in one of these boxes (or envelope), and meets the weight requirement (max 70 pounds) then you can use these boxes, and the price will be one flat rate for every zone. This makes it a lot easier figuring out how much to charge for shipping. The best part is they're FREE! Check out the Post Office's Store for FREE Supplies!