This is a tricky situation. I once had about 150 commissioned employees that faced the same problem. What I did when they did not have enough earnings to cover premiums was to send them a letter notifying them of the arrears and letting them know they had to make up the premiums within 90 days or be terminated from the plan for failure to make premium. They would be allowed pay premiums through Section 125 Plan any time a deduction was made.
If the employee defaulted twice in a 12 month running period they would be terminated from the plan and allowed to re-apply at open enrollment.
The carrier was aware of this arrangement and had no problem with it. As long as it was equally enforced, I was compliant with ERISA.