Hi nj123, we had a similar situation - we finally sent our application off 2 weeks ago. As we were sitting on the tram heading into the main post office, I asked my sponsor / boyfriend (common law partner) did you check the address? (he wrote the label). "Of course I did!" he replied.
So we posted the 1.5 kilo pack and headed home, glad to be finally done with it after 6 weeks of documents-collecting and stress.
No sooner had we got through the door than he made his way sneakily towards the PC while I made a cup of tea.
"Oh no! Oh no!" I heard him groaning from the living room.
I dashed back quickly.
"Oh no! Why didn't I listen to you?" he moaned. (I was so chuffed about him actually admitting that, that I nearly didn't hear what he said next)
"The PO box... it's 3000! I wrote 300!"
We leapt to our feet and were back in town faster than we'd ever got there before (with me laughing hysterically - it was either that or cry. The post office was closing in 10 minutes, and I couldn't believe this last-minute twist...)
Luckily we got the same guy immediately and he fetched the envelope from the sorting area so we could replace the label. "A few minutes later, and you'd have been too late!" he informed us.
The moral of the story - listen to your girlfriend / wife! Or, the moral my other half prefers: check the label before sending ;D