Need ideas for "proof" of sales for commerce BBs

We're finding that it is tricky to prove that you made a cash transaction for the following BBs:
Sell goods for $20 - sand level
Make $50 with a cottage industry - sand level
Sell goods for $200 - straw level
Make $500 with a cottage industry - straw level
$20,000 worth of work - wood level

Let's see if collectively we can build a list of scenarios that provide sufficient proof.  For example, to get my BB for selling $20 I had a picture of my farm stand table (with more than $20 worth of stuff) at two different church sales, photos after the goods were sold and a photo of a satisfied customer trading me money for a jar of syrup.

Proof for the $20 sand BB doesn't need to be as rock solid as the proof needed for Straw or Wood.  In my case above, for Straw or Wood I would have shown the rest of the crowd and a few more transaction pictures.  A time lapse video of the sales would have been perfect.

We want to ensure you can't easily fake the evidence or it will diminish the value of the BB for everyone and Otis won't be very impressed.  

Some transactions just won't be able to be proven to the level we need.  We figure about half of all transactions will be too elusive to prove sufficiently.

Here are some ideas of ways to provide adequate proof:
   - Time lapse video of selling stuff at your stand or booth
   - Pictures of the transactions showing goods and money changing hands combined with original inventory and ending inventory
   - Clear PayPal or other financial paper trail with matching photos

What are your ideas for a scenario that would provide enough proof???

I've been thinking about this and for me I sell a lot of stuff cash and about the same on a mobile app. Some stuff is invoiced.

For invoiced sales, a picture of the invoice and a picture of a bank statement showing the same transaction? (and the goods of course)
Last year we paid in several thousand $ worth of cash, I could show a picture of all the cash, a picture of the counting/bank receipt for payment and then again the bank statement showing the corresponding amount. the cash picture at least would be hard to fake. you need a lot of cash lying around in small change.
The mobile app is harder if one uses the business version there are printouts etc but of course they are all in Danish. the private version shows nothing and the money just dribbles into the bank account.

Equally Tax returns might be a possible way? Sales tax returns (where applicable)? this sounds especially valid when getting up to the 20k target that will leave a paper trail somewhere.

I keep detailed records of number of plants planted etc. perhaps receipts/pictures for raw materials? I have 3000 strawberry plants I certainly don't eat them all myself. If someone has shawn 20 sheep and spun the wool they're either wearing a lot of jumpers or they sold some.

Timelapse sounds great but how to do it? I can do right up to the wood level but that takes me 5 months, 5 months of 24/7 timelapse on a roadside stand (majority of sales) is going to take weeks to process and days to watch.  leaving a camera at an unattended stand is also going to mean one less camera, and markets have the same issue not to mention neither spot has power.
Would a sample day/sample market serve as proof for the entire thing?

I honestly think the 20k one will be easiest to prove as that amount of money leaves a tax trail.

Taxes would be a good bit of evidence.  Who'd cheat on their taxes in order to pay more, just to get a BB?