How do I configure a "Buy X credits get Y credits free" discount?

The best way to configure a discount that gives extra credits for free is to create a new credit package in Mariana Tek. The new credit package price will be the price you are advertising, but the number of credits will include the extra credits.
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​Example: "Buy 20 credits, get 2 free for $200!"
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If your normal 20 credit pack is $200, you will create a new one for the same price, but the credit quantity will be set for 22 rather than 20. Those 2 free credits will be built into the package so the customer gets them when they buy the package.