My advice is to keep original case receipts and approval letters in addition to what you're keeping.
In addition, it wouldn't hurt to scan everything you have and store in the cloud AND USB stick AND computer. Keeping three copies in three different places ensures you can recover data after natural disaster / fire or computer malfunction.
You never know when you may be asked again to reprove everything. The chance of it happening is low, but it would be hard if you destroy all evidence.
It's a lot of work to scan, but the cost of storing it in the cloud is free or close to 0.