Are you trying to enter that as a "custom item" at the ItsDeductible website? ItsDeductible will not allow a single item donation entry with a value of more than $500. It doesn't mean that you can't have a value over $500--only that ItsDeductible does not handle it. You can still use the TurboTax program to manually enter that item donation at tax preparation time.
I presume ItsDeductible doesn't work with a donation of an item valued at more than $500 because of the Form 8283 reporting requirements. When over $500, it requires additional information in columns e, f, and g of that form about when the item was acquired, how the item was acquired, and what the donor's basis in the item is. Those questions, among others, will be asked in the TurboTax program in order to prepare Form 8283.
Be aware that with a value of more than $500, additional recordkeeping requirements also come into play.
You can learn a lot more about charitable donations, including recordkeeping requirements, in this IRS Publication
https://www.irs.gov/publications/p526/ar02.htmlSpecifically, review the recordkeeping requirements for
"Deductions Over $500 But Not Over $5,000."