I made donations to a single charity, but many of my items are not found in itsdeducable. How do I add the items with the value?

ItsDeductible does allow you to customize contributions in  to the right of the Search by name.

For a list of acceptable values, search for estimated value ranges listed with any major charitable organization that accepts used items to resell.

In general, valuing donations at thrift store or garage sale value is commonly 10-25% of the purchase price for newer items in very good condition.