Techniques are described for facilitating interactions between computing
systems, such as via a third-party transaction authorization system that
automatically authorizes transactions between parties. In some
situations, the transactions are programmatic transactions involving the
use of fee-based Web services by executing application programs, with the
transaction authorization system authorizing and/or providing payments
for transactions between Web service provider and consumer parties in
accordance with private authorization instructions previously specified
by the parties. The authorization instructions may include predefined
instruction rule sets that regulate conditions under which a potential
transaction can be authorized, with the instruction rule sets each
referenced by an associated reference token. The transaction
authorization system can then receive a reference token from each party
to a potential transaction, and determine whether to authorize the
transaction based on whether the instruction rule sets associated with
the reference tokens are compatible or otherwise satisfied.