I've spent thousands at FCP and only had a few returns. But for a legitimate return, never had an issue. I thought it was no questions asked anyway?? I never call anyone but maybe in your case since it was not the correct part in the box, it would be different.

The downside of FCP I have found is this:
You have to buy the replacement part first. And the price is likely higher than when you purchased the original. You only get store credit for the original price.

You have to pay the shipping for the return. Radiators are expensive.. so you have to consider the cost of return shipping.

And then it's only a store credit, no option for a refund.

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2000 e39 M5 Titanium on Black
2006 e53 M54 Titanium on Black
2007 e83 Black on Black Sport pkg
2004 Avalanche 2500 lifted 4.88 gears
1994 jacked up Jeep
1987 GMC lifted short bed on tons