You don't need to amend. At this point, you need to write a letter to the IRS along with a list of donations or a spreadsheet explaining what you did and why you think the items are in different categories.
(It'sDeductible uses the same category for everything entered via ItsDeductible, that's just the way it is designed.)
The IRS requires that you have an appraisal for any single item "or group of similar items" with a value more than $5000. There is no black letter description of what constitutes a "group of similar items" and I have not seen any tax court cases on this. So I really can't tell you whether "used clothing", "used electronics", "use furniture" etc. would be accepted as being different categories, or one category "used household items."
You don't need to amend to change the descriptions, what you need to to is write a letter explaining your argument that you have items of different categories and each category is under $5000. You will need copies of your documentation; including information about how you acquired the items, the date you acquired them, the condition they were in, how you determined the fair market value, and a receipt from the organization. All these documentation requirements are listed in publication 526 for donations valued at more than $500 and you will be in a very weak audit position without them.
"ItsDeductible" values are not likely going to be accepted without some proof that the values used by ITsDeductible (which are generic nationwide values) actually correspond to values in your area. (Other customers have written in that their values were not accepted.) Proof would include proof of sales from eBay and local craigslist ads for similar items, and the prices charged in your local thrift stores. (For example, if ITsDeductible offers $10 for a sweater but the store you donated the sweater to tags it $5.99, you're going to have a problem holding up for that $10 value.)
In fact, you may find that a more defensible response is to claim slightly lower values so you get under the $5000 limit, if those values are more supported by your local market.
You may also be in trouble if you have a generic description from the charity "20 bags" or some such. For a large donation, ideally you will have your own itemized list and you will get someone from the charity to sign it to attest that you donated those specific items on that day. Otherwise, even with a well-documented list of items and their values, you also need to prove you donated the specific items on that list to the charity (and didn't just make up a list after the fact.)
Ultimately it will depend on the strength of your documents and the discretion of the examiner, whether you donated one group of similar items requiring an appraisal, or several different groups of items. In the end you have the right to appeal and take your case to tax court if you lose with the examiner. It will all depend on the quality of your documents surrounding how you acquired the property, how you determined the condition, and how you determined the value.
