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Former 'Bachelor' contestant Amanda Stanton is blowing it as a social media influencer ... so claims the luxury boutique that's suing her for failing to do the simplest task.

According to new legal docs ... Moonstone Vintage LA says it gave Amanda a very pricey gift -- a Chanel purse worth $3,249 -- and in exchange, she would pimp the store out to her 1.2 million Instagram followers. Pretty standard social media quid pro quo.

In the docs, obtained by TMZ, Moonstone claims Amanda failed on the pimping part. As they say, it ain't easy. The boutique says not only did Amanda fail to tag them in her post from Coachella ... but she kept the purse.

As a result, it's suing Amanda for $4,996 ... the cost of the purse, plus shipping and handling expenses.

Sources close to the situation tell TMZ ... Amanda requested the rare, vintage, black leather quilted mini backpack as a last-minute accessory for her Coachella trip earlier this year.

Our sources say the store personally delivered it from Santa Monica to the desert. And, while Amanda did post with the purse ... she tagged several other companies, but NOT Moonstone.

Adding insult to injury ... Amanda posted a second pic with the mini backpack and again didn't tag the company. Two days after her initial post, Amanda did finally tag Moonstone ... but the boutique's now saying too little, too late.