The verb «есть» (to eat) doesn't have a true perfective partner according to some sources. Its perfective counterpart would depend on the meaning:
So, when you mean consuming a certain "object", you use «съесть» to express that the object is fully eaten (within reason, it obviously doesn't have to be the seeds, core and all to 'count'!) However, when you are talking about the activity of eating as replenishing your energy, having a lunch break etc., use «пое́сть». In this skill we teach «съесть».
When counting, the Genitive plural form for «грамм» and «килограмм» may be either of the following:
The zero-ending option is definitely the most popular these days, at least in speech (it was considered colloquial about 40 years ago).
«Су́мка» is a bag made of a durable material. An expendable plastic or paper bag typical of supermarkets is «паке́т». And it is actually полиэтиле́новый (polyethylene), not пла́стиковый (which is usually used for hard plastics).
«Пакет» is also used for packs of sugar, salt, rice, milk etc.