Like everyone said...there are always options. If you NEED a part and only one place has it, either you pay to play or you wait until someone else gets it in stock.
But yeah, it's important to build up a stock of parts. I always order extras. And stuff I don't need at that moment. It's not bad to get that $.05 part you don't need when it's just taking onto another order...but it sucks when you NEED it and you spend $5 to ship it!
Got a pile of pop bumper skirts I may never need, but at least I won't be stuck scrambling!
Then again I just had a specific post I NEED for my MM. Nobody domestically has it. So one post that Marco normally sells for $3...um, it cost me almost $25 to get it from Australia.
But best to price shop. I know who has what for the best price.