Unless things have changed in the process, avoid doing visa extensions in Kabul if at all possible. visa extension are a serious pain in the ass, and involves days of bouncing back and forth between various ministries, offices, banks, etc.
It’s worth shopping around different consulates and embassies. the consulate in NYC issues 3 month visas (single entry only). there’s also consulates/embassies in Washington DC, LA, and Dubai that are worth checking out – Paris, I think Vienna, not sure where else.
Don’t know if the US consulates will accept a passport fedexed from overseas – they may tell you to apply from your home country, but I know foreign nationals can apply for visas at the NY consulate – you just have to be a legal resident/visitor.
Also worth checking out the visa expeditors like G3 or visaadvisors – they handle passport and visa services for various fees.
If all else fails, fly yourself from London to NYC, and do it in person. It will probably be more cost-effective and certainly more time-effective than flying from kandahar to kabul and dealing with it there.
May also vary based on your passport, so check that a UK (or other) passport can get the same visa options as US.
ps. also pls update this thread if you find more info. I’ve got people coming this way from various countries,at least one with a UK passport.