If you do something on a shoestring, you do it with a very small amount of money.
In earlier times, wandering peddlers offered townspeople a variety of items and trinkets, such as fabrics, embroidery materials, and even patent medicines. Another popular offering from these traveling salespeople was shoelaces. The fact that such vendors neither earned much money nor charged very much for their wares led to the connection of their literal shoestrings with a metaphorical application of “shoestring” to a very small amount of money. It’s still not uncommon to hear of a business being operated on a shoestring (even if these days it’s less likely that actual shoelaces are involved), but it’s also possible to speak of “traveling on a shoestring” and even “gardening on a shoestring.”