According to Guide (and I tested too), it could be submitted up to 30 items per transaction. But, the registrations form on my website ofers very often more then 30 items. Is there any solution for this?
Just have to deal with it. I'd say put in the overall order description something like "47 items, display limited to first 30" and then make sure to store the entire list of items in your database and pass the order ID along with the other information in case you want to look it up later.
If you really want to send all of your line item info to the credit card system:
You could break up your 1 transaction into multiple credit card transactions (with 30 line items per transaction).
But most merchants don't submit line item info to the credit card system:
Remember that the line item stuff is "new" for the credit card industry. Few merchants use it. Usually merchants maintain their own line-item invoices in their own coimputer systems, and only submit the net charge to the credit card system.
If you look at your credit card statement (as a customer), you'll see very few line item charges. I only see them for airline tix.
Well yes, but if you want to use just Authorize.net for tracking and not bother with your own layer on top, it's useful to have all that itemized info in the control panel. Not to mention it probably shows up in the automatic emails.