I was happy to learn that a birthday card with a few photos arrived in Barahona in three weeks.
So far I have sent four letters and only one did not arrive. It was a slightly oversize greeting card that had one of those musical chips in it. I suspect someone must have thought that the bulge was money or something else valuable and opened it.
I have found that making your letters look boring (normal size envelopes, normal colored ink used for printing the address improves the chances of it arriving.
I had several years of writing someone in Santiago and not a single letter was lost. That was before cellphones and I am sure there were more letters sent back then.