I have tried twice to send a postcard with an additional folded piece of paper on it, and both times it got lost, so I consider that experiment a failure. I don’t think I would try again.
When I want to write a lot on the card, I usually
choose a larger (15x17 cm) card
choose a smaller stamp
I don’t use stickers, or just one very small one
I write a draft of the text on the pc notepad. So that I can check how long it is and make some changes in order to shorten it if needed
Or, if the received asks to write a lot of things, I send the postcard inside an envelope with an additional note
My personal opinion is that too small handwriting is hard to read, even though I don’t have problems with my eyes. I use ballpoint pens 0.5 and 0.7 and I can write enough with them. Sometimes I write more, sometimes less, depending on the profile and how much I want to say things at that moment. Sometimes I prefer to fill the space with more decorations rather than writing. Everything goes. I like all styles.
This is a kind and thoughtful response. I had not considered this.
I really hope that people in these situations will explain on their profiles that (for example) they need the text to be large, or that they don’t want to receive cards in envelopes. I would always be happy to write fewer words in larger letters.
I have to confess I do sometimes cheat – especially as my handwriting is awful (I keep an arthritic spider in my biro!). So sometimes I actually print the message on a large address label and stick that to the card. It takes a bit of experimenting to get the size right, and to carefully cut the label to fit (after printing), but because it is so much clearer you can use a smaller font and write a longer message. At the beginning of the message I explain in a few words why I have done it this way.
I wanted to get back here after some great ideas and feedback on my initial post. Thanks a lot for this! There were helpful and inspiring things!
A few days ago I dared and started a first attempt with a small envelope on the back of the postcard as part of a postcard pen pal. The postcard hasn’t arrived yet (it has not traveled enough to have been delivered) so it remains exciting to see how this attempt will turn out in the end.
I’m excited and definitely had fun tinkering and experimenting. I’ll be happy to keep you up to date on how the postcard went on.
By the way, the postcard was sent from Germany to the USA.
It worked! The postcard with the small envelope on the back arrived at the recipient without any problems! This is fantastic and gives me completely new possibilities for the future when I have something more to report and want to put more text on the postcard.
I wonder if this would work in the USA, there is a thickness limit for postcards
USPS Postcard Size
To qualify as a postcard, a mail piece must be rectangular and meet these dimensions: At least 3-1/2” high X 5” long X 0.007” thick. No more than 4-1/4” high X 6” long X 0.016” thick. Have finished corners that do not ex USPS Postcard Size
although i havent used this technique specifically for writing more text in a card, i have covered the front with a piece of paper secured with washi tape on occasion to:
to protect the front of the postcard from being damaged by usps processing machinery (as some prefer not to receive envelopes)
or use as a small pocket to include bonus things like used stamps or stickers
or add a return address without sacrificing writing space (as described here)
and it has worked out successfully when sending from my USA location just make sure the addition is secured from all sides (so it doesnt get caught in mail processing machines) and isnt too bulky feeling or heavy
i love a good fully written postcard so i may try this technique so i can write more haha.
I’m quite new at this and I’ve learned when you send a postcard internationally the stamp is pretty big and the you have to put the air mail stamp on there. My thing is how are y’all making y’all cards pretty even sending to the USA here after you put stickers and stuff there really no room to write anything. My hand writing can get kind of messy with my mid age arthritis. Can I see your postcard for ideas.
What do you mean by air mail stamp? You have to only put stamps in the correct postage rate - international forever stamp or any other stamps adding up to $1.45 currently for sending internationally
I use ballpoint pens 0.5 and 0.7 and I can write enough with them. Sometimes I write more, sometimes less, depending on the profile and how much I want to say things at that moment. Sometimes I prefer to fill the space with more decorations rather than writing. Everything goes. I like all styles. 
There were helpful and inspiring things!
just make sure the addition is secured from all sides (so it doesnt get caught in mail processing machines) and isnt too bulky feeling or heavy
