In Russian we usually write “7” with an additional horizontal bar – it looks like ヲ. Moreover, the top line usually is not a line, but a wave 
I noticed that on postcards that I received, “7” is written just like it is typed – without this additional horizontal bar.
I wonder, if you receive a postcard with ID looks like “RU-238ヲ6”, would you understand that it is “RU-23876”? Or you would think that I wrote “7” by mistake and then struck out it, and the correct ID is “RU-2386”?
I know that here in the US we all write it as shown here “7” and our “1” is just one vertical line. I had a huge problem with this in Germany, because the woman’s head at the bank almost exploded since she thought my “17” was an “11.” Although when I wrote to my friends from Germany and did the 7 with a slash through it, they understood it was a 7, but thought I was just trying to be fancy. I know for postal workers you definitely won’t have a problem, because I’m sure they get those 7s all the time, so they’d know you weren’t crossing it out.
Oddly enough I brought back the German style of writing ones and sevens, and at a US bank, the teller laughed at me because the computer thought my 1s were sevens (since we don’t even write the little slash at the top). oh well =P
No, I don’t worry about postal workers My postcards usually reach they destination. But every card has an ID, and receiver have to register it. Sometimes I get e-mails with “somebody can’t find/understand the ID of your card”. But I always write an ID, and I always check if I wrote it correct. I wonder why receivers can’t understand my IDs…
You could always overkill it by writing out “seven.” For example, “RU-238seven6” haha but they might actually try and type that in instead of an actual 7 =P
Over here it’s totally whatever you want. We don’t hold to one way. When I was at high school I wrote with no horizontal line, now I do. Some people write a 1 like it is typed, like an ‘l’, or like a 1 with a horizontal line on the bottom. But again, we don’t stick to one way 
When I first came to the UK my “slashed 7s” were read as “4s” and my fancy 1 was read as 7. These days Britain copes well with a slashed 7, but the handwritten 1 largely remains a “I”.
I’ve seen both variants used, but the printed style is much more common here. Funny though that this problem occurs. I believe the reason 7s are sometimes written with with a horizontal line is to prevent them from being confused with 1s? We can never get it right…