Receiving from another country and customs

[SolarShiva]

So, up until now I've only sent the yellow 8x10 envelopes when requesting photos, this time I did a photo/document envelope as I REALLY don't want anything to happen to this photo. When I mailed it out yesterday my post office said I needed customs forms to send it priority since I wasn't/couldn't use the flat rate envelope to send it. The return envelope is 9.5 x 12.5 inches and I did put the regular, send to US from Canada, stamps on it, overpayed with them actually due to weighing it wrong the first time.

Now I'm concerned about customs..... I did buy some photos from overseas a couple years ago and it doesn't look like there were any customs forms included on the outside of that envelope and it was the same size as this return envelope... Do you think I'll be safe with what I did or do you think there will be a problem? I checked the customs info on the Canadian postal service website and it doesn't look like there will be a problem, also, the size of the envelope is included in their "letter post" description and letters don't require customs forms.... I haven't needed customs forms when sending the yellow 8x10s either, but that was in the past....

Thanks for any insight, hopefully good, you can give!