If you get the joke, you may be able to guess the theme. If you're still uncertain, the two codes hidden in this Vigenere cipher should help:
- EIQYLGTIMKUSVWDRSSWUBVFXMIFXYXLIAWZROHRYOJWMVFJBUGZLBOSK
Once you have the theme, complete the puzzle grid with the appropriate entries - there are of course many more, but the seven used here are at least in the correct order.
Puzzle grid
Use the values for A to H to find the cache at:
N H° F*(C-E) . G*(D+F)
W A° (G+E) . A (C-B)/F
Advance payment for the first consignment can be found with the cache. 
Advance payment - not to be sniffed at!
Congratulations on the FTF to =Andy_R.