Shoebox Urethane O-ring

Hi all. I bought my Shoebox 4500 back in 2011. I'm finally having to do maintenance on it on the lower cylinder. Can anyone tell me if there is another place to find/purchase the urethane o-rings? I know they are sold on the Shoebox website, but the item listed is $7.00 for six, and the shipping is $8.60. I think that's a little outrageous for such a small item which can fit in a regular envelope and weighs nothing. Please forgive me if it seems I'm nit-picking, I would just like to find a cheaper source.

Thanks.

Keith.
 
Are you sure it is a delrin o-ring and not a PTFE one? I ask just because, while o-rings are made of both, PTFE is vastly more common as an o-ring material while delrin is typically used for machined bushings, seats, etc., things which If it is a delrin o-ring, it'll be pretty hard to find, If you can pull specs for it though, a PTFE or seamless PTFE coated silicone o-ring could probably replace it and be had for a much lower price from the usual suspects. 

I say all this having only a basic understanding of how the shoebox works. I hope it is helpful though. I also hope someone with more shoebox-specific knowledge comes along and it able to just give you a link to a suitable replacement. 
 
Keith you bought your unit in 2011 and this is the first time you are rebuilding it or at least the lower cylindar never did anything else to it besides oil and grease ?


That is correct. I have never done any maintenance until now. I may not use it as much as others, but it has held up all this time. And each time I was still getting full fills at 4500 psi.

Keith.