Currently, there's no way to do this with just one campaign. Here is a workaround: use two campaigns at the same time.
Here's what we mean: you can assign different donation amounts to different products if you apply the Donate for Discount campaign for certain products, offering for example 20% off in exchange for a $10 donation. And for a separate set of products, you apply the % of Sales campaign, where you donate a % of proceeds or a fixed dollar amount from each sale of those items. This way, each campaign corresponds to a specific donation amount - in other words, you can't do two different % of Sales or Donate for Discount campaigns at once.
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