Installation:
Download and open up the zip file.  Upload the lang.custom_download_permission.php to the system/lang/english/ folder and the ext.custom_download_permission.php file to the system/extensions/ folder.  Got to ‘Admin- Utilities- Extensions Manager’ and enable the extension.

Settings:
image
1.  Set which fields require payment.  Overide - if checked, all file downloads protected by the download lock module will require a record of payment in the simple commerce module in order for a member to download.

Alternatively, you can select a field per field group that will be subjected to the ‘payment check’- any other fields will be subject to the regular settings you’re using in the download lock module.  For example, on this site, most files are available to download for any logged in member.  Pro files, which do require a payment record, are placed in the ‘Pro file’ custom field- only files in that field are checked against the simple commerce table.

2.  If there IS a recorded payment for the entry id associated with the file, then the file simply downloads as normal.  If not, what happens?  Well- if the ‘Denied message’ setting is left blank, the member is simply redirected to paypal just like if they used a ‘buy now’ link.  The ‘Denied message’ gives you the option of showing a message instead- and can accept most of the variables available to the simple commerce tag- mainly, {item_name}, {item_regular_price}, {buy_now_url}, etc.

3.  After successful payment, the buyer is redirected to another message screen- this one will show the contents specified in ‘Success message’- which allows you one variable- {download_link}.

Pretty simple, really!