You, yes – you! Give me your recommendations for tracking direct mail leads

I work as a freelance translator.

I'm starting a direct mail campaign to increase turnover.

I'd like to track my attempts/successes/failures in a trustworthy system, which (currently) only needs to be accessed by me.

What can you reccomend?

Good to know:

• Free > Cheap > Expensive
• Based in Germany (GDPR complinace is good)
• What can you recommend? I'm not currently interested in sending newsletters, mailing lists, selling products digitally, offering an online course, etc – I am sending out letters and I want to see which ones produced what results.

All suggestions gratefully received!