The ViaPost service allows you to send physical post direct from your computer to any address in the UK. The API lets you send mail, upload mailing lists, check account balances, and more. The API is intended for large batches (1,000 or more letters). For smaller batches, the Windows driver is probably more appropriate.
The following is a list of SDKs from our SDK directory that matched your search term. Though your definition of an SDK may differ, in our world, we define SDKs as platform- specific tools for consuming existing APIs of the sort we list in our API directory. For example, the Ruby SDK for consuming the Twitter Ads API. If you think an SDK, API, or other asset is missing from our directory, be sure to check our guidelines for making contributions to ProgrammableWeb.