What is an efficient method for sending or receiving over $100,000 online in a single transfer?

A wire transfer would be your best bet. Safe. Fixed price (regardless of how much amount you transfer) and its secure. Both parties can be confident of the success of the transaction if you use your bank. As Rob Rawson pointed out - its not a big deal. The only time you would really need to consider an outside company to do this transaction is if the transaction is being converted from US Dollars to some other currency, in which case you have to be cognisant of the exchange rate you would be getting. In such cases an FX broker may be able to beat the banks' standard FX conversion rate.