Mens American Flag Stars and Stripes G-String

You say this is very stretchy so if you are between sizes does the small / medium fit better ?

Approx sizing per manufacturer is: G-string sizing Small-Medium fits 28" - 34", Large-XL fits 34"-42"

Is this item, NPT-SW105-Small-Medium MENS AMERICAN FLAG STARS AND STRIPES G-STRING the same as NDS-JF-9311-USA-FLAG-SM VALENTINO'S MENS G-STRING BY NDS WEAR? I purchased the NPT-SW105 in the past but it had different bands then the Valentino’s G-string. I prefer the thinner bands that are on the Valentino’s g-string and want to make sure that I don’t get the same exact ones again.

Dear Paul, These are not the same type of string on these items, The Valentine has a rounded small string and the Flags and Stars has a thin but flat string around it.
We hope this is of assistance. Thank you

does this come with a c ring?

No it does not, this is a Flag Gstring only. 
You can see others here:
https://abcunderwear.com/pages/search-results-page?q=g-string+c-ring
https://abcunderwear.com/pages/search-results-page?q=c-ring

What waist sizes fit the small-medium? What waist sizes fit the large-xl?

Approx sizing per manufacturer is: Gstring sizing Small-Medium fits 28" - 34", Large-XL fits 34"-42";  Small-Medium 71-86cm, Large-XL 86-106cm

ID: NDS - JF - 9311 Is this item sold in WHITE and size LARGE ?

It does not, however we have a large selection of G-string in White or colors and you may find them all under our G-string section, here is a link for you:
https://abcunderwear.com/collections/mens-g-strings
This items is a 2 sizes item only due to the great fabric and stretch, so a Large would the the: Large-XL size you need to select. Hope this is of assistance.