I'll attest to the good customer service Revell have. They sent me replacement roof windows for the 1/48 Huey Hog I've got and also the older monogram version decals, no questions asked, didn't even ask for any money for the decals or proof I own the kit.
Although I think it depends on which Revell you deal with, because I think Revell USA is better for having parts in stock than RoG, or the other way around. I know I made an enquiry about the decals (the ones with the normal marines markings, not the ones for the red tailed Huey Hog) and one or the other told me to contact the other, probably RoG told me to contact Revell USA as they own what used to be Monogram (at least that's how I understand it).
Revell customer service seems way better than Italeri, I tried to blag some replacement 1/48 UH-1 roof windows for the ESCI kit I built (this was before I found out Fairy Power Spray works a treat for removing Tamiya clear green), and couldn't get anywhere with them, language barriers and the fact they wouldn't take anything other than a credit card payment were the main issues.
Airfix UK are slightly better than Italeri, but not as good as Revell imo. I wanted to get the RAF SAR decals from their Sea King kit (the yellow one) and whilst they supplied them they wanted a cheque for £5 or for me to call them and give my card details (presumably credit, not debit card) over the phone. Talk about operating in the stone age!!