HNE

I don't think they'll let you know they have received the extra documents. You could always email them and just ask for confirmation that they received the documents.
 
As for actual waiting time, I assume it depends on what information that asked for, why they needed it, etc. I'm pretty sure the embassy will have been closed at some point over Easter and perhaps even a few employees will have gone on holiday since it was the long 2 week break for schools so I wouldn't be surprised if it takes a little longer due to all that as well. 

We recently paid our I-130 fee via my husband's UK credit card (HSBC). We had no issues with payment and it was charged in USD. We did have an additional fee at our end which I think was about £11 or so because of the currency, but we expected that. As a side note, we never use our credit card so I half expected the charge to be flagged but it went through just fine so I wouldn't worry too much. It took the embassy about 5 or 6 days to charge the card.