Ask what you want to be good at
Retail and logistics are different disciplines. Running both well takes management attention that most growing stores would rather spend on product, staff and customers.
Volatility is the deciding factor
Internal fleets are efficient at steady volume and expensive at uneven volume. A partner absorbs the quiet weeks and the sudden busy ones.
You can change your mind later
Starting with a partner is also the lowest-cost way to learn what your delivery demand actually looks like before committing payroll to it.