How to avoid a small letter becoming a large letter.
1) Iron the folded contents flat.
2) Once the paper/card is in the envelope, Use the iron again and iron it very flat.
3) If you haven't got an iron handy and there's no breakables or other issues, hammer the envelope.
4) Otherwise, prepare everything a day or two in advance of posting and spend some time sitting on the envelopes to flatten them.
5) When you are sure all your mail is slimline and will easily slip through the wafer thin Post Office measuring template, stick stamps on it and post.