Personal opinion only, but I'm a fan of FCPEuro. Whenever I need parts, I tend to crosscheck 4 sites: FCP, IPD, eEuroparts, and Volvo Parts Webstore. I check for price usually, but if it's negligible I go for FCP due to their customer service and parts guarantees. I will say I've had a couple issues...

For a lot of smaller items - seals, gaskets, trim, hardware, other bits that are best to buy blue box - you might be surprised how reasonable pricing is at the dealer.