It means that if you decide to upgrade to a higher premium level, your cost will be decreased by that much. That value is a pro-rated one that will decrease through your membership term (so tomorrow or the day after it will be less, depending on rounding).
You have EC$13 of membership credit available for upgrades.
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I become a gold member recently and this message now appears to me :
You have EC$13 of membership credit available for upgrades.
What does it mean?