货到付款加费/ surcharge (COD) 3.6.0

Compatibility with PS9 added.

Minor bug on validate order in PS<1.7 solved.

• Compatibility with PS8.1 improved.

• Solved minor bug on admin order edit.

• Added filter by product
• Solved minor bug in PS 1.7.8.5

• Minor bug saving long category filter field in admin configuration page.