Online ordering question?

Why does it take the online stores longer to get parts? I am installing a oval dura-liner and DVL combination. After I placed my order with one of the online merchants that is often mentioned in these post, I was told I would experience a 3 week shipping delay. While I was waiting, I had a technical issue and called Simpson. In the conversation with the tech, I ask how far they were backed up. He did not know what I was talking about, and when I gave him a list of parts I needed, he said they were all in stock at the Mississippi warehouse. When my parts finally arrived, they had substituted a round tee for an oval tee. When I called to report it, I was told it would take at least an additional two weeks to get the part and that they would just refund my money. That same day, I ordered the oval tee from another online store that is often mentioned on here. I was told the part was in stock and would ship the next day. I have not received any shipping notice from them and they will not return my calls. I have called every dealer on the Simpson web site within a hundred miles and no one has the part in stock. I called Simpson to see if they had any dealers that stocked the part. The lady I spoke to told me that there was around 15 of these tees in the Mississippi warehouse and over 20 in the California warehouse, and that I could order it from Tractor Supply and it would be here in less than 4 days.
I'm setting here with $~1300 of liner and DVL and can't do anything till I get the tee at the bottom. Can anyone explain whats going on? Can Tractor Supply get parts faster than the online merchants? Winter is here, I have spent all my oil money on the stove, pipe and firewood, and am shut down because I can't get one part.

Well, I guess I panicked too soon, the part is up for delivery today!!

dynamitebuys.com was the first vendor. They were super nice. They shipped the DVL pipe I ordered the next day UPS. They apologized many times for the delay in the dura-liner shipping and it shipped the 2nd week of October, just as they said it would. When I called about the substituted tee, Nate apologized for the mistake and told me to just keep the part they had shipped in error and he would credit it back to my charge card. I checked online and it has been credited.
fireplacedeals.com was the other vendor. They were about $40.00 in total higher on the tee. I looked at several different pieces of the dura-liner and they appeared to be higher on everything, plus they added shipping charges. Although they did ship on time, I do wish customer service had done a better job of returning my calls.
Just for reference, tractor supply wanted $111.93 more than dynamitebuys.com for the same tee. I feel like I have saved almost half by ordering the pieces online vs. going through a retail store.