Health Fund Optical

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I was wondering if anyone knew much about health funds with their optical rebates?
From what I understand for dental work, if you go to providers by the health fund, you tend to get things cheaper and also certain work may not require any gap fee.
For optical items I think it is also the same but I want to ask if I can buy glasses online and still get my rebate? (I'm not worried about the gap fee)
One thing I do know if that there are certain optical stores online that are providers for certain health funds and so you can definitely get a rebate from buying glasses online but how about online non providers though?