The internal sutures used for suspension of your muscles and tissues (SMAS and platysma muscle) should not not be affected, and you are not likely to develop a hematoma (collection of blood) at 18 days following surgery. So you are very likely to be OK. HOWEVER, this type of question should ideally be posed to YOUR surgeon PRIOR to such an activity. Best of luck.
Each surgeon may have preferences as to what activity patients can do after facelift surgery, so it is best to check with your particular surgeon to see what activities you can do 2 weeks after your procedure. I do not believe that lifting 2 bags of 8 lbs. each would lead to any harm to your facelift results though.
I wil often tell patients limit exertion such as aerobics for at least 3-4 weeks and heavy lifting or more than 10-20 lbs for about 6 weeks. ALways best to ask your surgeon.
Although it is preferably to get definitive information from your Surgeon, lifting two bags of groceries at 18 days shouldn't present any problems. In my practice, prefer that my patients avoid cardiac exercise for 2 weeks and heavy weights (greater than 25 lbs), golf, tennis, etc. for 4 weeks after surgery.