Question regarding work experience

This is what I found written on the document checklist:

Self-employed individuals must provide documentation from 3rd party individual(s) indicating the service provided along with payment details. Self-declared main duties or affidavits are not acceptable evidence of work experience.

I owned my own cell-phones repair shop and I do not have any contracts or similar documents. I was the owner,manager, technician ,cleaner, basically everything. I was charging the customers when they would have come to take their phones.
However, there are two types of documents that exists:
1 The bill from the cash register. (It only has the cost of the service and it says "service"(no specified type of service like keypad problem or network problem etc.)
2 Recite issued when customer leaves his phone for service ( this was the proof that he have a phone at my shop). I had this recite returned to me after I return the phone to a customer.

I can also provide bank statements.

My question here is how do I prove what I had worked? (should I find some of my former customers to write a statement.)

Thanks