If you are experiencing this on SOP Sales Invoice posting edit lists, then maybe you are using drop ship items. Also perhaps you have the Shipping Notification Tool installed?
There is no posting hold as such on the document, the posting hold mechanism is being hijacked by the shipping tool to prevent posting of the document. It is actually because GP does not think the item in question have been shipped yet and so is preventing it from posting. Ensure that the item was marked as shipped using the ship notification window.
The setting in the INI file needs changing if you want to change this behaviour, details of the shipping notification window are in this post as is more information on the INI settings to change if you want to change the behaviour:
Dynamics GP Drop Shipping Sales invoices before purchase invoice with Shipment Notification Tool (SNT)