To investigate specific sales, in your Affiliate Admin > View/Download Earnings Report, you can find the Internal Transaction ID for each sale that earned you a commission. You should be able to provide this ID to your Merchant and ask them to look it up in their transaction logs, but it's up to them if they wish to comply with such a request and how much info they wish to provide, if any.
In the near future, we intend to support adding a parameter to the URL of each Affiliate link you post, which would be passed thru to the buyer's eventual order(s) and your notification/logs, so you could give every link a unique ID of your own and thus determine which sales resulted from which links you'd posted where.
Formal announcement of any new features would be posted to our Service Status and Updates forum category here:
...and if you're into RSS, we have a feed of updates to that forum here: