Trust versus operating
Unearned retainers belong in trust; earned fees in operating. Your processing setup should deposit each to the right account and charge all fees to operating.
Bar rules
State bars in the US and law societies in Canada publish guidance on card payments into trust. It generally requires that fees and chargebacks not reduce client funds. Read your own bar's guidance.
Retainers and plans
Retainers can be large; ACH is cheaper. Payment plans help clients and should be documented.
Pricing
Professional invoices are medium to large; interchange-plus and ACH matter most.