I don't know why the subscription payment is failing, but it is failing on the PayPal side. You would have to contact them to find out why. You should do it ASAP. I think the subscription will likely cancel itself after it fails the 3rd time. And that would be a shame because you're locked in at the old $4.98/year rate. If it cancels, you'll have to switch to manually paying the $15/year rate.
If PayPal can't help you, go to "My C3VR Membership" under the "My Account" menu here on C3VR's site to select your renewal method.
Good luck!
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-Adam Wartell
NCM Lifetime Member #1222
Founder: C3 Vette Registry
C4 Vette Registry, C6 Vette Registry
My first Vette, now owned by JB79: