That is not the correct analogy because the $15 product is not different to the first one. If you want to use the same analogy, here is what it sounds like: I want to sell you two products, $10 each, but he second one is optional and you have two choices.
Option 1: I give you 30% discount, so you pay me $7 for the first product and we are done with the deal. No second product involved.
Option 2: I give you 30% discount on the first product, so you pay $7 for it. For the second, we both have the option to buy/sell it. If I force you to buy, I will offer 50% discount so you pay me $5 but if you force me to sell, then I will only offer a 20% discount so you will have to pay $8.
Option 1: I give you 30% discount, so you pay me $7 for the first product and we are done with the deal. No second product involved.
Option 2: I give you 30% discount on the first product, so you pay $7 for it. For the second, we both have the option to buy/sell it. If I force you to buy, I will offer 50% discount so you pay me $5 but if you force me to sell, then I will only offer a 20% discount so you will have to pay $8.

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