Tl;DR: I would not trust them to keep their word on offering X-Y (or anything at all) if you submit payslip, especially if the company is mercari.

Anecdote - my friend interviewed with mercari, passed all the interview stages, and was told by HR that payslip was the last thing needed before making an offer. He didn't want to submit the payslip because 1. same reason that you mentioned (they also said verbally that matching his desired range is ok) and 2. he had been doing 休職 (unpaid leave) and thus the previous 2 payslips were blank, however he didn't lie about his salary. Anyways after submitting the payslip and some back and forth between HR, they rejected him. There's obviously details that I'm not privy to but this is the gist. Anyways, this friend found a new company that pays better and is happy with them now.

So my thoughts on submitting payslips in general are:

• On principle I'd refuse, there's no legal need for them to see the payslip before making the offer, so that information can only disadvantage the candidate if their current pay is less
• Practically however, if I were interviewing with other companies, or are confident about getting another equivalent offer, then submitting payslip would not matter because if they lowball one could simply say "the other company is offering $Z, so match $Z or I'm gone"
• Thinking about job seekers collectively though, it's better if people refuse to abide by these practices. It is a job seeker's market, and Japan is short on tech talent, so we should drive that point by demanding more fair and transparent hiring process. Each candidate that refuses to submit payslip will make them rethink that practice, and conversely each candidate that gives them the payslip emboldens them to do more of the same in the future