I can not understand what is the main meaning here?
Show people you're selling this product at a discount by setting a sale price in addition to the regular price.
From Facebook help.
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The part you have emboldened is not really a full clause in itself. The sentence would make more sense to you if you broke it up correctly. The first part tells you what to do:
Show people you're selling this product at a discount...
So it is telling you that if you are selling a product at a discounted price (ie the price you are selling it for is less than the recommended retail price) not to simply reduce the price, but also show people that you have.
How do you do this? The second part of the sentence answers:
...by setting a sale price in addition to the regular price.
It is suggesting that you should not only mark the item with your price but also show the full price so your customers can see how much discount they are getting.
OUR PRICE: £179.99!