Windscreen excess is normally far less than your insurance excess, normally around £75.
Windscreen cover is normally only on Full Comp policies, not TPFT ones.
A word of warning on this subject.
A mate was with the same company for 3 years.
On the second year, he had his windscreen put in from a stone on the M62.
Got it replaced by paying the excess (he is always fully comp) and thought nothing about it.
2 years later, he move companys and on the form ticked the "no claims in the last 5 years" box.
A month later, he had his insurance cancelled because a windscreen IS classed as a claim even though it doesn't affect your ncb
Nowdays, he is not a happy man because everytime he trys to renew, it comes up that he has had his insurance cancelled as is now classed as a very naughty boy who lies to insurance company's
He now gets shafted bigtime
The flip side is if you do declare it, they will bump the figure up anyway
My advise is if its only going to cost you a few quid more, sometimes its better to bit the bullet and pay for it yourself