<In some discussions on another forum, it has come to light that the same
part (same manufactuter, same part number), both on-line and in the
brick-and-mortar stores, is either $18 *or* $45 depending in which zip
code you live in (the on-line catalog has you enter your zip code before
searching for parts).

It was originally noticed as a East Coast-West Caost difference, but one
of the guys on the forum did the search for the same part with two zip
codes 30 miles apart and found the $18 vs $45 difference.  Dividing line
seems to be Kansas/Colorado.

I know that they are free to charge what they want to, but that's a 150%
difference.  That makes no sense.  Another example of MBA-think there?>

===============================================
to add to the posts:
I used to be a "wagon jobber", that translates to peddler in NJ during the
70's.
There were many interchangeable parts, such as ball joints, u joints,
ignition parts and so on.
BOP (buick, olds, pontiac) caddy and chevy all took the same ones in some
instances, but there were at least a 3 tier price list for them.
Of course caddy was the MOST EXPENSIVE, BOP next and of course CHEVY the
lowest. Then we got into the color of the sheet, from blue to gray and of
course discount from each one.
After doing this for many years, on the "wholesale level" there was no
bottom price! CASH, VOLUME, TIME OF MONTH (that is for 2% net 10) and
hitting a promised 100,000 spark plugs bought created a never ending place
to negotiate.
When I got to Oregon (da coast) and worked retail in a parts store, it was
hard to sell one fuse, explain how it works, where it goes, and then do the
paper work for the sale.
We did have "gypsy" parts OEM and OEM Original EQUIPMENT MFG and Original
Equivalency Mfg.
BTB DELCO was made by SES (springfield electronics) and you could get
anything made to your specs and boxed as you wanted.
Seems not much has changed BUT FOR CHINA supplying the CRAP now!

I got a response from PartsAmerica today and it almost agrees with you:

-------------------------------
PartsAmerica.com is affiliated with two auto part chains.  CSK Auto is our
supplier on the west coast and Advance Auto is our supplier on the east
coast.  Depending on your location will determine which auto part chain
supplies the part.  Since they are two different companies the pricing may
or may not be different for the same product.
-------------------------------

It looks like they ship from the closest supplier, regardless of price.  I
suppose in most cases the difference would be made up by savings on
shipping, but not in all cases.