Does the cost of standard flat shipping rate differ when an item is shipped to Hawaii?

I believe you are asking about Priority Flat Rate Envelopes and Boxes from the US Post Office. The page below from the USPS website states: Pay one flat rate to anywhere in the US. There is no mention of exceptions for Hawaii or Alaska. International rates are given on the page. Short of calling the Post Office to ask, it looks to me that shipping to Hawaii would be the same rate as other parts of the US.

Usually ground shipping is $8.00

No, there are no difference when you send stuff to Hawaii using the flat rate envelope/box then to any states in the Mainland. The delivery time (priority mail) is the same as in mainland also. The recipient will receive the package on the 3rd day.
For the most updated rate, check with www.usps.com.
Have a good one!

No. If you are using the flat-rate shipping envelopes or boxes from the USPS, they will cost the same whether you are mailing next door or all the way across country (including Hawaii & Alaska). Flat-rate envelopes cost $4.60 & the flat-rate boxes (both sizes) cost $8.95. Be sure that the packaging specifically states "flat-rate" as they provide other size boxes also.

You can go to www.usps.com and they will give you rates to shipping all over the world and the different ways you can ship items..

yes it differs 'cause standard shipping is ground and the only way to send things to Hawaii is by air. We haven't built the bridge to Hawaii yet. ;-}