muffler

My 98 GMC Jimmy has had a rattle in the exhaust for a couple years. Now its getting worse and I am going to have a new muffler installed. I took it to a
local shop. They can put one on that meets original specifications or a universal muffler. My concern is engine damage or fuel economy if the universal has more back pressure. The universal is $69 verses $99.
Money is short not worked in over a year. Social Security as is true with 80% of the claims has denied me and I have to appeal it with an attorney and that will take another 1 or 2.
My question is will the cheaper universal muffler cause engine damage or reduce MPG.
Thanks
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