You don't need a full scan or a giant CSV file. You can do it with a single small file in a single directory. The file contains the state of a pseudo random number generator. On each request, generate a number, save the new PRNG state into a new temp file in a subdirectory, and rename the temp file over top of the original file. If renaming fails, try the whole process a few more times. This allows multiple processes to serve requests concurrently.

Exercise for the reader: avoid re-reading the PRNG state file on retries.

If they expire in 5 days, that's 1/6 of a million.

You don't really need anything relational, so I think sqlite is overkill, and since multiple locations presumably need to write to it, you'd either have to use something like Litestream or write a coordination server to manage the updates from the mail intake locations

I think a Redis instance would be better. Assuming each mail intake device has a unique ID, and ID 0 is intended for un-redeemed postzegelcodes, you could have newly created postzegelcodes SETNX a key with value 0 and a TTL of 5 days, then when mail is being scanned, WATCH the key in a multi transaction, and if the value is 0, SET it to the ID of the intake device, while removing the TTL. The mail can then be rejected at intake if the transaction failed. Upon mail delivery then, you clear the key, which frees it for later re-use.