If you bulk edit your listings, you may be able to toggle through the options to quickly turn off OBO from them at once and repeat as necessary.

FedEx is almost always accurate with their cost estimate following through to the end billable on my monthly invoice. There have been a few instances where package shape caused a penalty fee, but 9.5/10 are accurate from my experience. If it's not accurate and there is no penalty for an error in my packaging, the additional charge is usually minute.

I try to stick with First Class packages for any under 16 oz shipment. If over, I usually list my item with "standard shipping" if I'm not sure if USPS Priority Mail or FedEx Ground will be cheaper. This gives me full control over shopping for the best shipping rate instead of forcing myself to use a service that may be cost prohibitive with respect to the sale price/destination. It may cause items to sell a little slower if competition offers faster service, but I prefer customers who aren't in a rush to get something where they'll be more relaxed about other elements of the post-sale process as well.

I also don't have a strong sensitivity to the shipping since I get most of my inventory for free or near free. I'm never close to losing money on a sale unless there's a quality issue.