you cannot get what some expeditors can provide from government
different agencies have different minimum times for expeditors
again, there is discussion of using expeditors on FT

there is not a separate line for walk ins at washington agency

my experience >
2011 - appointment 9 days out, traveling 14 days out, got passport same day

2015 - walk in, traveling 5 days out, picked up passport 2 days later
many walk ins including many same day travel - some picked up same day
after checking in, they give you a number with a letter, re letters >
seems like 2 priority, appointment, walk in same day, other walk in