Freight increase email?

I recently received an email from a seller asking me to pay an additional $6 for shipping after I already paid $12 when I placed my order. They claim the original shipping channel is unavailable and need to switch to a different one, increasing the shipping fee.

However, according to their website's shipping policy:

• All shipping fees are calculated automatically at checkout based on weight and location.
• Shipping fees must be paid in full and in advance.
• No mention of additional charges after checkout (except for customs fees in Europe, which don't apply here).

Since I already paid the shipping fee upfront as required, I'm wondering if this extra charge is legitimate or if I should push back. Has anyone dealt with something similar? Should I be concerned about a possible scam, or is this a common issue? I've also already received my tracking number.

Thanks!