Most of my clients now are virtual.  If they are comfortable with uploading to a online portal, they are comfortable with paying online.

For the not-so-virtual clients (including those who mail their information to me), I ask them if they plan on paying electronically for any payments.  Each year I have my clients fill out an annual questionnaire of yes/no questions (such as "did your martial status change", etc.).  If you do something similar, maybe include a question asking if they owe money (including estimates) if they plan on paying electronically or not.