I have received (snail/postal) mail from Domain Registry of America asking me to renew (and transfer if you read closely) my domain. Who are they and why are they sending me mailings?

DRA (Domain Registry of America) is using very carefully (one might say deceptively) worded mass mailings to domain owners trying to get them to transfer/switch their domain management to DRA. Unless you desire to have DRA be your registrar of record, disregard any mail from them. If you registered your domain through QWK.net, your registrar of record will be QWK.net/Tucows. QWK.net is a reseller of Tucows domain registry service due to the high cost of being ICANN certified to be a registrar of record directly.

For what it is worth, these DRA mailings border on fraud and Network Solutions/Verisign has already been prosecuted for similar activities in the past. It is likely that DRA is aware of Verisign's prosecution and has worded their mailings to be one step this side of legal (check your local laws). The practice is known as domain slamming and is akin to and named after long distance service slamming that takes place in the telephone industry.