The heroine is not good at sewing -- or spinning, or weaving, but it's sewing that comes up. She explains that with prosperous peasants, the bride can give you cloth of her weaving, or thread, when you write down her marriage contract. (Said contracts being a big deal in the the era. Especially since peasants tend to think they are being cheated if they understand more than two-thirds of the words that are used.) But among the poor, likely the only thing she can give you is to mend your clothes, or sew up some of that cloth for you.
There have always been enough poor peasant brides that she's farmed out all her sewing and mending. No practice makes no perfection.
Whereupon I started to ponder the exchange rate and whether you would have this bride sew the bodice and that one the sleeves, or whether the bride would have to sew up several shirts for an equitable exchange.
True there are a fair number of professional letter writers in her world, and they go on circuits to supply villages with anything that needs writing. (Mostly letters, indeed. While the illiterates in this world are more likely to need a marriage contract than a letter, those that send letters seldom send only one in a lifetime.) On the other hand, they are fairly lengthy circuits because you drain a village dry of all jobs rather quickly, so it qualifies as a rare skill, and you can't count on having another one coming by.
On the third hand, even if I give her some magical assistance, there's a real limit to the amount of payment in kind she can lug about. There's a constraint right there.