When your product uses products from other parties you need to make sure your First Line Support process can handle this. As soon as it is clear that the reported issue is caused by a third party product you need to transfer the issue to that party. This is easier said than done!
Let's take an example which makes it directly clear how difficult it can be. Open source product Birt, a reporting tool, provides lovely functionality. When you depend upon the Birt community to get a problem in Birt fixed you could sometimes wait for ages. This will not help you getting a solution (on time) for your customer. For such products you need to make sure your Support and Maintenance contract with your customer covers this.When you use a third party product supported by support and maintenance contract you are better covered. Make sure your customer is aware of your dependence. For such products you need to make sure your Support and Maintenance contract with your customer covers this as well. Only garantee support when your verdor supplies it.
Another option is a third party product for which your customer should have a Support and Maintenance Contract with that supplier. The transfer of reported support issue should go, preferable, via your customer. Make sure you help your customer understand the issue is not located in your product. Provide evidence to help him convince the third party supplier.