I think that używalność is the ease of use in human language (also known as usability). Transakcyjność here refers to the ability to conduct transactions with the cards, which in human language refers the number of locations accepting the cards.
In a larger sense, this is the number of ways (physical stores, on-line) that transactions can be conducted with the cards.

This is especially so since the text refer to promotion, not the number of transactions that the bank can boast of.

Zawsze mówię, że proste jest piękne... :-)
(A i nieźle się gugluje.)