I live in Australia and can't look myself except through Scotland's People which is expensive.
You can access historical Scottish certificates far less expensively than Australian ones.
It costs £1.50 to view a digital image of a historical certificate on Scotland's People. That, at today's exchange rate, is AUD$2.78.
A New South Wales certificate costs AUD$39 - 13 times the cost of a Scottish one
A Victoria certificate costs AUD$22.50 - just under 7 times the cost of a Scottish one
A Queensland certificate costs AUD$24.70 - over 7 times the cost of a Scottish one
A Western Australia certificate costs AUD$20 - less than 6 times the cost of a Scottish one
A South Australia certificate costs AUD$36.25 - 12 times the cost of a Scottish one
And you get your Scottish image instantly - you don't have to wait for it to come by post.
If you need a formal certificate to prove a relationship, or if you want a recent*, non-historical certificate, a Scottish one does cost £12 and you do have to wait for it to come by post. £12 is AUD$23.85 so it is still cheaper than a certificate from NSW, QLD or SA and similar to the cost of a VIC or WA one.
*Less than 100 years for births, 75 years for marriages and 50 years for deaths.
I don't know which Church Mary married in
She almost certainly didn't get married in a church building at all. Weddings were mostly held in the bride's home or, if she had no parents' home or she married a long way from home, in her employer's house or in the parish manse.
It could have been the UP Church
Her first marriage could not have been celebrated by a UP minister as the UP Church did not exist in 1844. The United Presbyterian Church was formed in 1847 by the union of the United Secession Church and the Relief Church.
Her 1855 marriage certificate should tell you exactly where she was married and by what religious denomination.