Luckily, most insurance policies allow for permissive use drivers, so the damage you caused should be covered under your grandmother's policy as long as you aren't a full-time driver. Unfortunately, your grandmother's insurance rate is likely to increase after the claim since it will be considered an At Fault accident. Our State of Insurance report
shows that an At Fault accident will have an average impact of about $581 in additional premium per year for 3 years
so that's something to consider. As far as the repair facility is concerned, the other driver should have a choice to get the vehicle repaired at any location they prefer. If you aren't able to afford the repair costs out of your own pocket then your only other choice would be to file a claim.