As a business, we need to keep some records "forever". This generally would be a 'book of first entry', which in the case of shoppers would be something like spreadsheets or calendars or whatever you consistently use to list your jobs, fees and receipts (as in payments received). Paper receipts from shops indeed fade and these days we no longer need to mail in receipts (yes, we had to mail them in a decade and more ago). Since we now submit them digitally, it makes sense to retain those digital receipts and after a suitable period of time, destroy the paper ones.
If you read your ICAs, you will find that different MSCs have different time periods for you to retain records. One company I used to work with required 2 years as they might be needed to document a legal action, while another indicated you could toss them once you were paid. It is not now, and never has been, my intent to revisit my receipts and collateral to determine what I can toss next month, what I can toss in 6 months, what I need to retain for 2 years, etc. A quarterly file (or monthly if you are collecting lots of receipts and collateral) is an easy way to keep the material accessible yet out of the way. When the files age a bit, I bundle them and throw them in a banker's box and when the box gets full, pitch the oldest bundles--which by then are 5-6 years old, which handles IRS requirements as well. About once a year my credit union has a 'shred day' and that is the time when I gather old financial records and old shop materials, shove it all in a box, watch it get dumped into the shredder and I am returned my box. That also assures that any confidential materials I may have printed from the MSC have also been discretely disposed of.
All of my digital records--both personal and business--are retained digitally and backed up to external drives with some frequency. 16 years of Mystery Shopping digital records--including narratives saved in Word, some shop instructions in pdf, site photos, digital recordings, spreadsheets, and digitized receipts all save handily on a small thumb drive, so it is not like I am wasting a lot of computer space.
Each of us has to organize our materials in a way that is intuitively 'correct' to ourselves because each of us is going to need to be able to locate our records if and when needed. I will not store my records on someone else's server (i.e. 'The Cloud') because there are potential ownership issues of those materials. My computer 'file system' is purely a 'KISS' system: A folder called "Mystery Shopping" that at its root has my spreadsheet for each year I have been doing this and the spreadsheet of companies with whom I am registered including user name and login (among other information). The folder then has subfolders for each company. At the root of that subfolder are the current year's work and folders for previous years' work. At the end of the year I create a folder for the year ending and move all loose documents into that so that I am ready for the new year's work for that MSC. For me, that is intuitive. For someone else that would likely be onerous.